Boths PCs in the same exact location? If so we can discount interference or poor strength. The USB 2 vs 3 is a valid point - along the same train of thought your PC's USB controllers might be flakey (not my first gut instinct, tho.)
Your DHCP, TCP, DNS and other settings could create packet loss.
If the two conputers were at different desks and there's more than 1 wifi around looking into interference might yield results. If the two PCs are connecting to different wifis there's a whole heap of possibilities... I'd need a better picture of what's been tried and what's going on to be of much more use.