I recall back when I was on XP, I bought a USB headset from Radio Shack. (Gigaware)
After using it for a while, I found that one of my two cores on my CPU was running at 100%.
This was reproduceable.
I concluded that the in-built USB headset driver (I never installed any drivers), was buggy, and started eating kernel CPU time after a while.
I don't know if they ever fixed the bugs in the Win7 USB headset driver, or if they have the same bugs.
If it does have the same bug, I could easily see how it could give you lag.