It could also be a driver problem. Make a quick check with Process Explorer
and see whether you have a lot of Deferred Procedure Calls (DPC). In PE you see it as 2nd or 3d entry. If you are under 1%, you are OK. 2 to 4% and higher would be a problem.
Then you can also run the Latency Checker
. If that shows red bars, something is wrong. It might even tell you which driver is the culprit.