One thing to keep in mind is that with USB devices, the CPU does
all the work in managing the traffic.
I have seen CPU usage numbers for USB ethernet run steadily at
about 35% with peaks at 60% on a 2.4 GHz P4 - even though the
computer owner's broadband connection was only 2 Mbps.
He was very frequently getting slow network speeds when the CPU
was too busy to give the USB ethernet the attention it needed.
Conversely, if the CPU was giving too many cpu cycles to the USB
ethernet, then performance in other apps really suffered and the
system was unresponsive to user input.
Since this guy liked to play multi-player games over the www,
having the network connection eat up so much of his CPU's
frequently meant stuttering performance in the games.
Conversely, if he gave the game a higher priority, then not
enough CPU cycles were left for USB ethernet.
Installing a real NIC into a PCI slot solved that user's
problems. A $30 NIC did far more for his gaming performance than
his $400 video card upgrade. CPU usage by the PCI NIC seldom
hits 4%.
