One thing to consider, is the source and quality of your USB device driver. Many moons ago when evaluating a 8-port serial card (under OS/2), we found that every additional concurrent transfer halved the overall combined throughput until by the time we had all 8 ports transferring data concurrently, the combined throughput was almost at a crawl.

As we were potentially going to be purchasing many hundreds of these cards, the manufacturers sent us a support technician to help us resolve the problem. Turned out that the install had selected to use a generic serial port driver rather than the manufacturer supplied one. And as that had been written to deal with just the standard 2-port configuration, it was overly conservative in its design and that was the cause of the abysmal throughput.

Once we installed the serial DD designed to deal with the 8-port device, things improved immensely. I'd strongly suggest that you visit the web site for whoever manufactured your USB card and check the availability of the latest drivers. Also, if you installed anything involved in the USB stick using Plug&Pray, check that it used the manufacturers DDs and not generic ones. Especially if your copy of Windows is relatively old. Early builds shipped with only USB v1.x drivers, but most devices work much better with later v2.x drivers.

The availability and capabilities of DDs always trail the hardware.