Unfortunately, a big part of the problem might be USB. Unless both
your computer and the capture device are USB-2, USB just can't handle
data fast enough to do full frame-rate captures.
There are some things to check though. Make sure before you start the
capture that the ONLY things running in your computer are Explorer and
Systray. If anything else is running it will slow down your system. You
should also make sure your drives have DMA enabled.