To be honest you are absolutely right. I have the same problem and it took me ages to work out what was going wrong. I had to use dpc lat to find out. In the end I discovered it was the Dolby Headphone mode that cause all the deferred procedure call latency. I deactivated it and I have been happy since. Sometime it returns though, however a comp restart sorts it out. By the way I have a Xonar Essence ST. Just to prove it still affects all of their range.
By the way thank you TooTay. Previously I had been looking for a solution to the DPC problem. Don't know how long this has been out but 4 months ago there was none. Thank you again, may be now I can enjoy Dolby Headphone in game, as it was a really nice mode for fps games.