SD Card performance with different read buffer size ( SYS_FS_FileRead )

[ajefr]

Hello,
I'm facing a strange behaviour while using the SYS_FS_FileRead function.
I have one file to read.
If I read it with a 512 byte buffer, read time is correct (70 seconds).
If I read it with a smaller buffer, read time increase a little bit (77, 80 secs).
But if I try with a 1024 bytes buffer, the read time is catastrophic : maybe 20 minutes !

code is a "do while end of file" with this read function :
nBytesRead = SYS_FS_FileRead( fileHandle, (void*)read_buffer, buff_size);
buff_size was tested with 256 / 512 / 1024
Obviously, read_buffer has the correct size for handling the bytes amount.

How can it be possible ?

Thanks