Not directly related to the OP, but since it's a popular setup: the best you can hope for with a USB 2.0 FTDI or USB serial converter is USB fullspeed frame rat

Not directly related to the OP, but since it's a popular setup: the best you can hope for with a USB 2.0 FTDI or USB serial converter is USB fullspeed frame rate, or 1 kHz. So 1 millisecond in one direction.