Well the USB port can only handle so much data at one time. If both are plugged into a hub, then the bandwith available for each device is cut. (5 ports is 20% bandwith). I believe that is how it works. So 480Mb/s for USB2 and 12Mb/s for USB1 divided by 5 is 96Mb/s and 2Mb/s respecively.
I guess I should ask, are they both plugged into the same hub?