I suspect that its the performance of your usb connection. Depending on the hardware design, device driver software etc you can get widely varying performance of the USB subsystem. On a pure hardware basis it may be very high capbability but implementation details such as I/O architecture, hardware buffering, latency, can make a big difference. ie not much to do with the processor.
When you run your comint software is it processor intensive? That may not really be a problem as long as the memory/processor/storage is fast enough.
Are you writing this comint software?