Hello,
There may be a number of factors due to the nature of USB to take into consideration. If there is more USB traffic then the OS and USB have to schedule the communication with each of the USB devices.
It’s highly dependent on the OS and USB Host and is out with our control.
FTDI devices use USB Bulk transfers and those are designed to transfer large amounts of data with error-free delivery, but with no guarantee of bandwidth.
You could try disconnecting all other USB device to see if it has an effect.
You may also want to try and reduce the latency timer.
The default latency timer is 16ms when using the FTDI drivers.
The latency timer can only be set to 1ms for our USB High Speed products (like FT2232H).
You can read more about this in
Data Throughput, Latency & Handshaking.
The other option that you might want to explore is by not using LibMPSSE.
You could try using the D2xx drivers direct and implement your own SPI communication.
See
AN_108 Command Processor For MPSSE and MCU Host Bus Emulation Modes which should help.
Also there may be other FTDI Community users who might be able to help you further.
Best Regards,
FTDI Community