I was experiencing exactly the same issue after updating to fall creators update on windows 10. It was driving me nuts.
I tried removing game bar and xbox DVR components from windows to see if that would help, but with little result. Running i5 6600k and gtx 1080 - hadn't experienced any issues before the update.
Followed your method of changing processor affinity then changing back and I got back to smooth 60fps! Registered on here so I could say thanks!
Have you found the fix to be permanent or do you need to do this every time you play?
