diff --git a/src/ophyd_async/core/__init__.py b/src/ophyd_async/core/__init__.py
index 418d4bd0ff..ff4d6f8be9 100644
--- a/src/ophyd_async/core/__init__.py
+++ b/src/ophyd_async/core/__init__.py
@@ -83,6 +83,7 @@
     DEFAULT_TIMEOUT,
     CalculatableTimeout,
     Callback,
+    LazyMock,
     NotConnected,
     Reference,
     StrictEnum,
@@ -176,6 +177,7 @@
     "DEFAULT_TIMEOUT",
     "CalculatableTimeout",
     "Callback",
+    "LazyMock",
     "CALCULATE_TIMEOUT",
     "NotConnected",
     "Reference",
diff --git a/src/ophyd_async/core/_device.py b/src/ophyd_async/core/_device.py
index 1fe7855f3d..eb43abff58 100644
--- a/src/ophyd_async/core/_device.py
+++ b/src/ophyd_async/core/_device.py
@@ -3,17 +3,15 @@
 import asyncio
 import sys
 from collections.abc import Coroutine, Iterator, Mapping, MutableMapping
+from functools import cached_property
 from logging import LoggerAdapter, getLogger
 from typing import Any, TypeVar
-from unittest.mock import Mock
 
 from bluesky.protocols import HasName
 from bluesky.run_engine import call_in_bluesky_event_loop, in_bluesky_event_loop
 
 from ._protocol import Connectable
-from ._utils import DEFAULT_TIMEOUT, NotConnected, wait_for_connection
-
-_device_mocks: dict[Device, Mock] = {}
+from ._utils import DEFAULT_TIMEOUT, LazyMock, NotConnected, wait_for_connection
 
 
 class DeviceConnector:
@@ -37,25 +35,23 @@ def create_children_from_annotations(self, device: Device):
         during ``__init__``.
         """
 
-    async def connect(
-        self,
-        device: Device,
-        mock: bool | Mock,
-        timeout: float,
-        force_reconnect: bool,
-    ):
+    async def connect_mock(self, device: Device, mock: LazyMock):
+        # Connect serially, no errors to gather up as in mock mode
+        for name, child_device in device.children():
+            await child_device.connect(mock=mock.child(name))
+
+    async def connect_real(self, device: Device, timeout: float, force_reconnect: bool):
         """Used during ``Device.connect``.
 
         This is called when a previous connect has not been done, or has been
         done in a different mock more. It should connect the Device and all its
         children.
         """
-        coros = {}
-        for name, child_device in device.children():
-            child_mock = getattr(mock, name) if mock else mock  # Mock() or False
-            coros[name] = child_device.connect(
-                mock=child_mock, timeout=timeout, force_reconnect=force_reconnect
-            )
+        # Connect in parallel, gathering up NotConnected errors
+        coros = {
+            name: child_device.connect(timeout=timeout, force_reconnect=force_reconnect)
+            for name, child_device in device.children()
+        }
         await wait_for_connection(**coros)
 
 
@@ -67,9 +63,8 @@ class Device(HasName, Connectable):
     parent: Device | None = None
     # None if connect hasn't started, a Task if it has
     _connect_task: asyncio.Task | None = None
-    # If not None, then this is the mock arg of the previous connect
-    # to let us know if we can reuse an existing connection
-    _connect_mock_arg: bool | None = None
+    # The mock if we have connected in mock mode
+    _mock: LazyMock | None = None
 
     def __init__(
         self, name: str = "", connector: DeviceConnector | None = None
@@ -83,10 +78,18 @@ def name(self) -> str:
         """Return the name of the Device"""
         return self._name
 
+    @cached_property
+    def _child_devices(self) -> dict[str, Device]:
+        return {}
+
     def children(self) -> Iterator[tuple[str, Device]]:
-        for attr_name, attr in self.__dict__.items():
-            if attr_name != "parent" and isinstance(attr, Device):
-                yield attr_name, attr
+        yield from self._child_devices.items()
+
+    @cached_property
+    def log(self) -> LoggerAdapter:
+        return LoggerAdapter(
+            getLogger("ophyd_async.devices"), {"ophyd_async_device_name": self.name}
+        )
 
     def set_name(self, name: str):
         """Set ``self.name=name`` and each ``self.child.name=name+"-child"``.
@@ -97,28 +100,33 @@ def set_name(self, name: str):
             New name to set
         """
         self._name = name
-        # Ensure self.log is recreated after a name change
-        self.log = LoggerAdapter(
-            getLogger("ophyd_async.devices"), {"ophyd_async_device_name": self.name}
-        )
+        # Ensure logger is recreated after a name change
+        if "log" in self.__dict__:
+            del self.log
         for child_name, child in self.children():
             child_name = f"{self.name}-{child_name.strip('_')}" if self.name else ""
             child.set_name(child_name)
 
     def __setattr__(self, name: str, value: Any) -> None:
+        # Bear in mind that this function is called *a lot*, so
+        # we need to make sure nothing expensive happens in it...
         if name == "parent":
             if self.parent not in (value, None):
                 raise TypeError(
                     f"Cannot set the parent of {self} to be {value}: "
                     f"it is already a child of {self.parent}"
                 )
-        elif isinstance(value, Device):
+        # ...hence not doing an isinstance check for attributes we
+        # know not to be Devices
+        elif name not in _not_device_attrs and isinstance(value, Device):
             value.parent = self
-        return super().__setattr__(name, value)
+            self._child_devices[name] = value
+        # ...and avoiding the super call as we know it resolves to `object`
+        return object.__setattr__(self, name, value)
 
     async def connect(
         self,
-        mock: bool | Mock = False,
+        mock: bool | LazyMock = False,
         timeout: float = DEFAULT_TIMEOUT,
         force_reconnect: bool = False,
     ) -> None:
@@ -133,26 +141,39 @@ async def connect(
         timeout:
             Time to wait before failing with a TimeoutError.
         """
-        uses_mock = bool(mock)
-        can_use_previous_connect = (
-            uses_mock is self._connect_mock_arg
-            and self._connect_task
-            and not (self._connect_task.done() and self._connect_task.exception())
-        )
-        if mock is True:
-            mock = Mock()  # create a new Mock if one not provided
-        if force_reconnect or not can_use_previous_connect:
-            self._connect_mock_arg = uses_mock
-            if self._connect_mock_arg:
-                _device_mocks[self] = mock
-            coro = self._connector.connect(
-                device=self, mock=mock, timeout=timeout, force_reconnect=force_reconnect
+        if mock:
+            # Always connect in mock mode serially
+            if isinstance(mock, LazyMock):
+                # Use the provided mock
+                self._mock = mock
+            elif not self._mock:
+                # Make one
+                self._mock = LazyMock()
+            await self._connector.connect_mock(self, self._mock)
+        else:
+            # Try to cache the connect in real mode
+            can_use_previous_connect = (
+                self._mock is None
+                and self._connect_task
+                and not (self._connect_task.done() and self._connect_task.exception())
             )
-            self._connect_task = asyncio.create_task(coro)
-
-        assert self._connect_task, "Connect task not created, this shouldn't happen"
-        # Wait for it to complete
-        await self._connect_task
+            if force_reconnect or not can_use_previous_connect:
+                self._mock = None
+                coro = self._connector.connect_real(self, timeout, force_reconnect)
+                self._connect_task = asyncio.create_task(coro)
+            assert self._connect_task, "Connect task not created, this shouldn't happen"
+            # Wait for it to complete
+            await self._connect_task
+
+
+_not_device_attrs = {
+    "_name",
+    "_children",
+    "_connector",
+    "_timeout",
+    "_mock",
+    "_connect_task",
+}
 
 
 DeviceT = TypeVar("DeviceT", bound=Device)
diff --git a/src/ophyd_async/core/_mock_signal_backend.py b/src/ophyd_async/core/_mock_signal_backend.py
index 878313e051..43fb2ae7df 100644
--- a/src/ophyd_async/core/_mock_signal_backend.py
+++ b/src/ophyd_async/core/_mock_signal_backend.py
@@ -1,13 +1,13 @@
 import asyncio
 from collections.abc import Callable
 from functools import cached_property
-from unittest.mock import AsyncMock, Mock
+from unittest.mock import AsyncMock
 
 from bluesky.protocols import Descriptor, Reading
 
 from ._signal_backend import SignalBackend, SignalDatatypeT
 from ._soft_signal_backend import SoftSignalBackend
-from ._utils import Callback
+from ._utils import Callback, LazyMock
 
 
 class MockSignalBackend(SignalBackend[SignalDatatypeT]):
@@ -16,7 +16,7 @@ class MockSignalBackend(SignalBackend[SignalDatatypeT]):
     def __init__(
         self,
         initial_backend: SignalBackend[SignalDatatypeT],
-        mock: Mock,
+        mock: LazyMock,
     ) -> None:
         if isinstance(initial_backend, MockSignalBackend):
             raise ValueError("Cannot make a MockSignalBackend for a MockSignalBackend")
@@ -34,11 +34,14 @@ def __init__(
 
         # use existing Mock if provided
         self.mock = mock
-        self.put_mock = AsyncMock(name="put", spec=Callable)
-        self.mock.attach_mock(self.put_mock, "put")
-
         super().__init__(datatype=self.initial_backend.datatype)
 
+    @cached_property
+    def put_mock(self) -> AsyncMock:
+        put_mock = AsyncMock(name="put", spec=Callable)
+        self.mock().attach_mock(put_mock, "put")
+        return put_mock
+
     def set_value(self, value: SignalDatatypeT):
         self.soft_backend.set_value(value)
 
@@ -46,7 +49,7 @@ def source(self, name: str, read: bool) -> str:
         return f"mock+{self.initial_backend.source(name, read)}"
 
     async def connect(self, timeout: float) -> None:
-        pass
+        raise RuntimeError("It is not possible to connect a MockSignalBackend")
 
     @cached_property
     def put_proceeds(self) -> asyncio.Event:
diff --git a/src/ophyd_async/core/_mock_signal_utils.py b/src/ophyd_async/core/_mock_signal_utils.py
index 30d48dbfe0..08976a0468 100644
--- a/src/ophyd_async/core/_mock_signal_utils.py
+++ b/src/ophyd_async/core/_mock_signal_utils.py
@@ -2,17 +2,26 @@
 from contextlib import asynccontextmanager, contextmanager
 from unittest.mock import AsyncMock, Mock
 
-from ._device import Device, _device_mocks
+from ._device import Device
 from ._mock_signal_backend import MockSignalBackend
-from ._signal import Signal, SignalR, _mock_signal_backends
+from ._signal import Signal, SignalConnector, SignalR
 from ._soft_signal_backend import SignalDatatypeT
+from ._utils import LazyMock
+
+
+def get_mock(device: Device | Signal) -> Mock:
+    mock = device._mock  # noqa: SLF001
+    assert isinstance(mock, LazyMock), f"Device {device} not connected in mock mode"
+    return mock()
 
 
 def _get_mock_signal_backend(signal: Signal) -> MockSignalBackend:
-    assert (
-        signal in _mock_signal_backends
+    connector = signal._connector  # noqa: SLF001
+    assert isinstance(connector, SignalConnector), f"Expected Signal, got {signal}"
+    assert isinstance(
+        connector.backend, MockSignalBackend
     ), f"Signal {signal} not connected in mock mode"
-    return _mock_signal_backends[signal]
+    return connector.backend
 
 
 def set_mock_value(signal: Signal[SignalDatatypeT], value: SignalDatatypeT):
@@ -45,12 +54,6 @@ def get_mock_put(signal: Signal) -> AsyncMock:
     return _get_mock_signal_backend(signal).put_mock
 
 
-def get_mock(device: Device | Signal) -> Mock:
-    if isinstance(device, Signal):
-        return _get_mock_signal_backend(device).mock
-    return _device_mocks[device]
-
-
 def reset_mock_put_calls(signal: Signal):
     backend = _get_mock_signal_backend(signal)
     backend.put_mock.reset_mock()
diff --git a/src/ophyd_async/core/_signal.py b/src/ophyd_async/core/_signal.py
index 22f4ffdc57..d4d4d7ffe3 100644
--- a/src/ophyd_async/core/_signal.py
+++ b/src/ophyd_async/core/_signal.py
@@ -4,7 +4,6 @@
 import functools
 from collections.abc import AsyncGenerator, Awaitable, Callable, Mapping
 from typing import Any, Generic, cast
-from unittest.mock import Mock
 
 from bluesky.protocols import (
     Locatable,
@@ -30,9 +29,14 @@
 )
 from ._soft_signal_backend import SoftSignalBackend
 from ._status import AsyncStatus
-from ._utils import CALCULATE_TIMEOUT, DEFAULT_TIMEOUT, CalculatableTimeout, Callback, T
-
-_mock_signal_backends: dict[Device, MockSignalBackend] = {}
+from ._utils import (
+    CALCULATE_TIMEOUT,
+    DEFAULT_TIMEOUT,
+    CalculatableTimeout,
+    Callback,
+    LazyMock,
+    T,
+)
 
 
 async def _wait_for(coro: Awaitable[T], timeout: float | None, source: str) -> T:
@@ -54,26 +58,28 @@ class SignalConnector(DeviceConnector):
     def __init__(self, backend: SignalBackend):
         self.backend = self._init_backend = backend
 
-    async def connect(
-        self,
-        device: Device,
-        mock: bool | Mock,
-        timeout: float,
-        force_reconnect: bool,
-    ):
-        if mock:
-            self.backend = MockSignalBackend(self._init_backend, mock)
-            _mock_signal_backends[device] = self.backend
-        else:
-            self.backend = self._init_backend
+    async def connect_mock(self, device: Device, mock: LazyMock):
+        self.backend = MockSignalBackend(self._init_backend, mock)
+
+    async def connect_real(self, device: Device, timeout: float, force_reconnect: bool):
+        self.backend = self._init_backend
         device.log.debug(f"Connecting to {self.backend.source(device.name, read=True)}")
         await self.backend.connect(timeout)
 
 
+class _ChildrenNotAllowed(dict[str, Device]):
+    def __setitem__(self, key: str, value: Device) -> None:
+        raise AttributeError(
+            f"Cannot add Device or Signal child {key}={value} of Signal, "
+            "make a subclass of Device instead"
+        )
+
+
 class Signal(Device, Generic[SignalDatatypeT]):
     """A Device with the concept of a value, with R, RW, W and X flavours"""
 
     _connector: SignalConnector
+    _child_devices = _ChildrenNotAllowed()  # type: ignore
 
     def __init__(
         self,
@@ -89,14 +95,6 @@ def source(self) -> str:
         """Like ca://PV_PREFIX:SIGNAL, or "" if not set"""
         return self._connector.backend.source(self.name, read=True)
 
-    def __setattr__(self, name: str, value: Any) -> None:
-        if name != "parent" and isinstance(value, Device):
-            raise AttributeError(
-                f"Cannot add Device or Signal {value} as a child of Signal {self}, "
-                "make a subclass of Device instead"
-            )
-        return super().__setattr__(name, value)
-
 
 class _SignalCache(Generic[SignalDatatypeT]):
     def __init__(self, backend: SignalBackend[SignalDatatypeT], signal: Signal):
diff --git a/src/ophyd_async/core/_soft_signal_backend.py b/src/ophyd_async/core/_soft_signal_backend.py
index d0e48c7212..ba21c3ba9b 100644
--- a/src/ophyd_async/core/_soft_signal_backend.py
+++ b/src/ophyd_async/core/_soft_signal_backend.py
@@ -4,6 +4,7 @@
 from abc import abstractmethod
 from collections.abc import Sequence
 from dataclasses import dataclass
+from functools import lru_cache
 from typing import Any, Generic, get_origin
 
 import numpy as np
@@ -90,6 +91,7 @@ def write_value(self, value: Any) -> TableT:
             raise TypeError(f"Cannot convert {value} to {self.datatype}")
 
 
+@lru_cache
 def make_converter(datatype: type[SignalDatatype]) -> SoftConverter:
     enum_cls = get_enum_cls(datatype)
     if datatype == Sequence[str]:
diff --git a/src/ophyd_async/core/_utils.py b/src/ophyd_async/core/_utils.py
index db4afae04a..ca20d90a3c 100644
--- a/src/ophyd_async/core/_utils.py
+++ b/src/ophyd_async/core/_utils.py
@@ -14,6 +14,7 @@
     get_args,
     get_origin,
 )
+from unittest.mock import Mock
 
 import numpy as np
 
@@ -120,20 +121,29 @@ async def wait_for_connection(**coros: Awaitable[None]):
 
     Expected kwargs should be a mapping of names to coroutine tasks to execute.
     """
-    results = await asyncio.gather(*coros.values(), return_exceptions=True)
-    exceptions = {}
+    exceptions: dict[str, Exception] = {}
+    if len(coros) == 1:
+        # Single device optimization
+        name, coro = coros.popitem()
+        try:
+            await coro
+        except Exception as e:
+            exceptions[name] = e
+    else:
+        # Use gather to connect in parallel
+        results = await asyncio.gather(*coros.values(), return_exceptions=True)
+        for name, result in zip(coros, results, strict=False):
+            if isinstance(result, Exception):
+                exceptions[name] = result
 
-    for name, result in zip(coros, results, strict=False):
-        if isinstance(result, Exception):
-            exceptions[name] = result
-            if not isinstance(result, NotConnected):
+    if exceptions:
+        for name, exception in exceptions.items():
+            if not isinstance(exception, NotConnected):
                 logging.exception(
                     f"device `{name}` raised unexpected exception "
-                    f"{type(result).__name__}",
-                    exc_info=result,
+                    f"{type(exception).__name__}",
+                    exc_info=exception,
                 )
-
-    if exceptions:
         raise NotConnected(exceptions)
 
 
@@ -252,3 +262,38 @@ def __init__(self, obj: T):
 
     def __call__(self) -> T:
         return self._obj
+
+
+class LazyMock:
+    """A lazily created Mock to be used when connecting in mock mode.
+
+    Creating Mocks is reasonably expensive when each Device (and Signal)
+    requires its own, and the tree is only used when ``Signal.set()`` is
+    called. This class allows a tree of lazily connected Mocks to be
+    constructed so that when the leaf is created, so are its parents.
+    Any calls to the child are then accessible from the parent mock.
+
+    >>> parent = LazyMock()
+    >>> child = parent.child("child")
+    >>> child_mock = child()
+    >>> child_mock()  # doctest: +ELLIPSIS
+    <Mock name='mock.child()' id='...'>
+    >>> parent_mock = parent()
+    >>> parent_mock.mock_calls
+    [call.child()]
+    """
+
+    def __init__(self, name: str = "", parent: LazyMock | None = None) -> None:
+        self.parent = parent
+        self.name = name
+        self._mock: Mock | None = None
+
+    def child(self, name: str) -> LazyMock:
+        return LazyMock(name, self)
+
+    def __call__(self) -> Mock:
+        if self._mock is None:
+            self._mock = Mock(spec=object)
+            if self.parent is not None:
+                self.parent().attach_mock(self._mock, self.name)
+        return self._mock
diff --git a/src/ophyd_async/epics/adcore/_single_trigger.py b/src/ophyd_async/epics/adcore/_single_trigger.py
index 9fd81b413d..165204d371 100644
--- a/src/ophyd_async/epics/adcore/_single_trigger.py
+++ b/src/ophyd_async/epics/adcore/_single_trigger.py
@@ -19,7 +19,8 @@ def __init__(
         **plugins: NDPluginBaseIO,
     ) -> None:
         self.drv = drv
-        self.__dict__.update(plugins)
+        for k, v in plugins.items():
+            setattr(self, k, v)
 
         self.add_readables(
             [self.drv.array_counter, *read_uncached],
diff --git a/src/ophyd_async/epics/core/_pvi_connector.py b/src/ophyd_async/epics/core/_pvi_connector.py
index 812e4ec473..1c5c0eceb6 100644
--- a/src/ophyd_async/epics/core/_pvi_connector.py
+++ b/src/ophyd_async/epics/core/_pvi_connector.py
@@ -1,7 +1,5 @@
 from __future__ import annotations
 
-from unittest.mock import Mock
-
 from ophyd_async.core import (
     Device,
     DeviceConnector,
@@ -11,6 +9,7 @@
     SignalRW,
     SignalX,
 )
+from ophyd_async.core._utils import LazyMock
 
 from ._epics_connector import fill_backend_with_prefix
 from ._signal import PvaSignalBackend, pvget_with_timeout
@@ -64,29 +63,29 @@ def _fill_child(self, name: str, entry: Entry, vector_index: int | None = None):
             backend.read_pv = read_pv
             backend.write_pv = write_pv
 
-    async def connect(
-        self, device: Device, mock: bool | Mock, timeout: float, force_reconnect: bool
+    async def connect_mock(self, device: Device, mock: LazyMock):
+        self.filler.create_device_vector_entries_to_mock(2)
+        # Set the name of the device to name all children
+        device.set_name(device.name)
+        return await super().connect_mock(device, mock)
+
+    async def connect_real(
+        self, device: Device, timeout: float, force_reconnect: bool
     ) -> None:
-        if mock:
-            # Make 2 entries for each DeviceVector
-            self.filler.create_device_vector_entries_to_mock(2)
-        else:
-            pvi_structure = await pvget_with_timeout(self.pvi_pv, timeout)
-            entries: dict[str, Entry | list[Entry | None]] = pvi_structure[
-                "value"
-            ].todict()
-            # Fill based on what PVI gives us
-            for name, entry in entries.items():
-                if isinstance(entry, dict):
-                    # This is a child
-                    self._fill_child(name, entry)
-                else:
-                    # This is a DeviceVector of children
-                    for i, e in enumerate(entry):
-                        if e:
-                            self._fill_child(name, e, i)
-            # Check that all the requested children have been filled
-            self.filler.check_filled(f"{self.pvi_pv}: {entries}")
+        pvi_structure = await pvget_with_timeout(self.pvi_pv, timeout)
+        entries: dict[str, Entry | list[Entry | None]] = pvi_structure["value"].todict()
+        # Fill based on what PVI gives us
+        for name, entry in entries.items():
+            if isinstance(entry, dict):
+                # This is a child
+                self._fill_child(name, entry)
+            else:
+                # This is a DeviceVector of children
+                for i, e in enumerate(entry):
+                    if e:
+                        self._fill_child(name, e, i)
+        # Check that all the requested children have been filled
+        self.filler.check_filled(f"{self.pvi_pv}: {entries}")
         # Set the name of the device to name all children
         device.set_name(device.name)
-        return await super().connect(device, mock, timeout, force_reconnect)
+        return await super().connect_real(device, timeout, force_reconnect)
diff --git a/src/ophyd_async/plan_stubs/_ensure_connected.py b/src/ophyd_async/plan_stubs/_ensure_connected.py
index d4835b710c..0ad5cff518 100644
--- a/src/ophyd_async/plan_stubs/_ensure_connected.py
+++ b/src/ophyd_async/plan_stubs/_ensure_connected.py
@@ -1,13 +1,11 @@
-from unittest.mock import Mock
-
 import bluesky.plan_stubs as bps
 
-from ophyd_async.core import DEFAULT_TIMEOUT, Device, wait_for_connection
+from ophyd_async.core import DEFAULT_TIMEOUT, Device, LazyMock, wait_for_connection
 
 
 def ensure_connected(
     *devices: Device,
-    mock: bool | Mock = False,
+    mock: bool | LazyMock = False,
     timeout: float = DEFAULT_TIMEOUT,
     force_reconnect=False,
 ):
diff --git a/src/ophyd_async/tango/base_devices/_base_device.py b/src/ophyd_async/tango/base_devices/_base_device.py
index 2227d5ddbc..1f98c4bd4f 100644
--- a/src/ophyd_async/tango/base_devices/_base_device.py
+++ b/src/ophyd_async/tango/base_devices/_base_device.py
@@ -1,9 +1,9 @@
 from __future__ import annotations
 
 from typing import TypeVar
-from unittest.mock import Mock
 
 from ophyd_async.core import Device, DeviceConnector, DeviceFiller
+from ophyd_async.core._utils import LazyMock
 from ophyd_async.tango.signal import (
     TangoSignalBackend,
     infer_python_type,
@@ -117,41 +117,42 @@ def create_children_from_annotations(self, device: Device):
             list(self.filler.create_signals_from_annotations(filled=False))
             self.filler.check_created()
 
-    async def connect(
-        self, device: Device, mock: bool | Mock, timeout: float, force_reconnect: bool
-    ) -> None:
-        if mock:
-            # Make 2 entries for each DeviceVector
-            self.filler.create_device_vector_entries_to_mock(2)
+    async def connect_mock(self, device: Device, mock: LazyMock):
+        # Make 2 entries for each DeviceVector
+        self.filler.create_device_vector_entries_to_mock(2)
+        # Set the name of the device to name all children
+        device.set_name(device.name)
+        return await super().connect_mock(device, mock)
+
+    async def connect_real(self, device: Device, timeout: float, force_reconnect: bool):
+        if self.trl and self.proxy is None:
+            self.proxy = await AsyncDeviceProxy(self.trl)
+        elif self.proxy and not self.trl:
+            self.trl = self.proxy.name()
         else:
-            if self.trl and self.proxy is None:
-                self.proxy = await AsyncDeviceProxy(self.trl)
-            elif self.proxy and not self.trl:
-                self.trl = self.proxy.name()
-            else:
-                raise TypeError("Neither proxy nor trl supplied")
-
-            children = sorted(
-                set()
-                .union(self.proxy.get_attribute_list())
-                .union(self.proxy.get_command_list())
-            )
-            for name in children:
-                # TODO: strip attribute name
-                full_trl = f"{self.trl}/{name}"
-                signal_type = await infer_signal_type(full_trl, self.proxy)
-                if signal_type:
-                    backend = self.filler.fill_child_signal(name, signal_type)
-                    backend.datatype = await infer_python_type(full_trl, self.proxy)
-                    backend.set_trl(full_trl)
-                    if polling := self._signal_polling.get(name, ()):
-                        backend.set_polling(*polling)
-                        backend.allow_events(False)
-                    elif self._polling[0]:
-                        backend.set_polling(*self._polling)
-                        backend.allow_events(False)
-            # Check that all the requested children have been filled
-            self.filler.check_filled(f"{self.trl}: {children}")
+            raise TypeError("Neither proxy nor trl supplied")
+
+        children = sorted(
+            set()
+            .union(self.proxy.get_attribute_list())
+            .union(self.proxy.get_command_list())
+        )
+        for name in children:
+            # TODO: strip attribute name
+            full_trl = f"{self.trl}/{name}"
+            signal_type = await infer_signal_type(full_trl, self.proxy)
+            if signal_type:
+                backend = self.filler.fill_child_signal(name, signal_type)
+                backend.datatype = await infer_python_type(full_trl, self.proxy)
+                backend.set_trl(full_trl)
+                if polling := self._signal_polling.get(name, ()):
+                    backend.set_polling(*polling)
+                    backend.allow_events(False)
+                elif self._polling[0]:
+                    backend.set_polling(*self._polling)
+                    backend.allow_events(False)
+        # Check that all the requested children have been filled
+        self.filler.check_filled(f"{self.trl}: {children}")
         # Set the name of the device to name all children
         device.set_name(device.name)
-        return await super().connect(device, mock, timeout, force_reconnect)
+        return await super().connect_real(device, timeout, force_reconnect)
diff --git a/tests/core/test_device.py b/tests/core/test_device.py
index 2fc127f17b..39a9b70a5d 100644
--- a/tests/core/test_device.py
+++ b/tests/core/test_device.py
@@ -1,4 +1,5 @@
 import asyncio
+import time
 import traceback
 from unittest.mock import Mock
 
@@ -174,43 +175,50 @@ def __init__(self, name: str) -> None:
         super().__init__(name)
 
 
+@pytest.mark.parametrize("parallel", (False, True))
+async def test_many_individual_device_connects_not_slow(parallel):
+    start = time.time()
+    bundles = [MotorBundle(f"bundle{i}") for i in range(100)]
+    if parallel:
+        for bundle in bundles:
+            await bundle.connect(mock=True)
+    else:
+        coros = {bundle.name: bundle.connect(mock=True) for bundle in bundles}
+        await wait_for_connection(**coros)
+    duration = time.time() - start
+    assert duration < 1
+
+
 async def test_device_with_children_lazily_connects(RE):
     parentMotor = MotorBundle("parentMotor")
 
     for device in [parentMotor, parentMotor.X, parentMotor.Y] + list(
         parentMotor.V.values()
     ):
-        assert device._connect_task is None
+        assert device._mock is None
     RE(ensure_connected(parentMotor, mock=True))
 
     for device in [parentMotor, parentMotor.X, parentMotor.Y] + list(
         parentMotor.V.values()
     ):
-        assert (
-            device._connect_task is not None
-            and device._connect_task.done()
-            and not device._connect_task.exception()
-        )
+        assert device._mock is not None
 
 
-@pytest.mark.parametrize("use_Mock", [False, True])
-async def test_no_reconnect_signals_if_not_forced(use_Mock):
+async def test_no_reconnect_signals_if_not_forced():
     parent = DummyDeviceGroup("parent")
 
-    connect_mock_arg = Mock() if use_Mock else True
-
-    async def inner_connect(mock, timeout, force_reconnect):
+    async def inner_connect(mock=False, timeout=None, force_reconnect=False):
         parent.child1.connected = True
 
     parent.child1.connect = Mock(side_effect=inner_connect)
-    await parent.connect(mock=connect_mock_arg, timeout=0.01)
+    await parent.connect(mock=False, timeout=0.01)
     assert parent.child1.connected
     assert parent.child1.connect.call_count == 1
-    await parent.connect(mock=connect_mock_arg, timeout=0.01)
+    await parent.connect(mock=False, timeout=0.01)
     assert parent.child1.connected
     assert parent.child1.connect.call_count == 1
 
     for count in range(2, 10):
-        await parent.connect(mock=connect_mock_arg, timeout=0.01, force_reconnect=True)
+        await parent.connect(mock=False, timeout=0.01, force_reconnect=True)
         assert parent.child1.connected
         assert parent.child1.connect.call_count == count
diff --git a/tests/core/test_signal.py b/tests/core/test_signal.py
index b03f3b42c5..f80e21f084 100644
--- a/tests/core/test_signal.py
+++ b/tests/core/test_signal.py
@@ -9,11 +9,7 @@
 from bluesky.protocols import Reading
 
 from ophyd_async.core import (
-    DEFAULT_TIMEOUT,
     DeviceCollector,
-    MockSignalBackend,
-    NotConnected,
-    Signal,
     SignalR,
     SignalRW,
     SoftSignalBackend,
@@ -32,80 +28,44 @@
 )
 from ophyd_async.core import StandardReadableFormat as Format
 from ophyd_async.epics.core import epics_signal_r, epics_signal_rw
-from ophyd_async.plan_stubs import ensure_connected
 
 
 def num_occurrences(substring: str, string: str) -> int:
     return len(list(re.finditer(re.escape(substring), string)))
 
 
+def test_cannot_add_child_to_signal():
+    signal = soft_signal_rw(str)
+    with pytest.raises(
+        AttributeError,
+        match="Cannot add Device or Signal child foo=<.*> of Signal, "
+        "make a subclass of Device instead",
+    ):
+        signal.foo = signal
+
+
 async def test_signal_connects_to_previous_backend(caplog):
     caplog.set_level(logging.DEBUG)
-    int_mock_backend = MockSignalBackend(SoftSignalBackend(int), Mock())
-    original_connect = int_mock_backend.connect
-    times_backend_connect_called = 0
-
-    async def new_connect(timeout=1):
-        nonlocal times_backend_connect_called
-        times_backend_connect_called += 1
-        await asyncio.sleep(0.1)
-        await original_connect(timeout=timeout)
-
-    int_mock_backend.connect = new_connect
-    signal = Signal(int_mock_backend)
-    await asyncio.gather(signal.connect(), signal.connect())
+    signal = soft_signal_rw(int)
+    mock_connect = Mock(side_effect=signal._connector.backend.connect)
+    signal._connector.backend.connect = mock_connect
+    await signal.connect()
+    assert mock_connect.call_count == 1
+    assert num_occurrences(f"Connecting to {signal.source}", caplog.text) == 1
+    await asyncio.gather(signal.connect(), signal.connect(), signal.connect())
+    assert mock_connect.call_count == 1
     assert num_occurrences(f"Connecting to {signal.source}", caplog.text) == 1
-    assert times_backend_connect_called == 1
 
 
 async def test_signal_connects_with_force_reconnect(caplog):
     caplog.set_level(logging.DEBUG)
-    signal = Signal(MockSignalBackend(SoftSignalBackend(int), Mock()))
+    signal = soft_signal_rw(int)
     await signal.connect()
     assert num_occurrences(f"Connecting to {signal.source}", caplog.text) == 1
     await signal.connect(force_reconnect=True)
     assert num_occurrences(f"Connecting to {signal.source}", caplog.text) == 2
 
 
-async def test_signal_lazily_connects(RE):
-    class MockSignalBackendFailingFirst(MockSignalBackend):
-        succeed_on_connect = False
-
-        async def connect(self, timeout=DEFAULT_TIMEOUT):
-            if self.succeed_on_connect:
-                self.succeed_on_connect = False
-                await super().connect(timeout=timeout)
-            else:
-                self.succeed_on_connect = True
-                raise RuntimeError("connect fail")
-
-    signal = SignalRW(MockSignalBackendFailingFirst(SoftSignalBackend(int), Mock()))
-
-    with pytest.raises(RuntimeError, match="connect fail"):
-        await signal.connect(mock=False)
-
-    assert (
-        signal._connect_task
-        and signal._connect_task.done()
-        and signal._connect_task.exception()
-    )
-
-    RE(ensure_connected(signal, mock=False))
-    assert (
-        signal._connect_task
-        and signal._connect_task.done()
-        and not signal._connect_task.exception()
-    )
-
-    with pytest.raises(NotConnected, match="RuntimeError: connect fail"):
-        RE(ensure_connected(signal, mock=False, force_reconnect=True))
-    assert (
-        signal._connect_task
-        and signal._connect_task.done()
-        and signal._connect_task.exception()
-    )
-
-
 async def time_taken_by(coro) -> float:
     start = time.monotonic()
     await coro
diff --git a/tests/epics/demo/test_demo.py b/tests/epics/demo/test_demo.py
index 838b9f5811..e29e13bf29 100644
--- a/tests/epics/demo/test_demo.py
+++ b/tests/epics/demo/test_demo.py
@@ -10,6 +10,7 @@
 
 from ophyd_async.core import (
     DeviceCollector,
+    LazyMock,
     NotConnected,
     assert_emitted,
     assert_reading,
@@ -198,9 +199,10 @@ async def test_retrieve_mock_and_assert(mock_mover: demo.Mover):
 
 
 async def test_mocks_in_device_share_parent():
-    mock = Mock()
-    async with DeviceCollector(mock=mock):
-        mock_mover = demo.Mover("BLxxI-MO-TABLE-01:Y:")
+    lm = LazyMock()
+    mock_mover = demo.Mover("BLxxI-MO-TABLE-01:Y:")
+    await mock_mover.connect(mock=lm)
+    mock = lm()
 
     assert get_mock(mock_mover) is mock
     assert get_mock(mock_mover.setpoint) is mock.setpoint
diff --git a/tests/plan_stubs/test_ensure_connected.py b/tests/plan_stubs/test_ensure_connected.py
index 62cc7d034b..4665ddcee9 100644
--- a/tests/plan_stubs/test_ensure_connected.py
+++ b/tests/plan_stubs/test_ensure_connected.py
@@ -31,8 +31,8 @@ def connect():
     device2 = MyDevice("PREFIX2", name="device2")
 
     def connect_with_mocking():
-        assert device2.signal._connect_task is None
+        assert device2.signal._mock is None
         yield from ensure_connected(device2, mock=True, timeout=0.1)
-        assert device2.signal._connect_task.done()
+        assert device2.signal._mock is not None
 
     RE(connect_with_mocking())