Use a CPU and GPU (HDD and ram might be an idea too) monitor program and log them in the back ground for say 10mins (if the game is slow from boot or wait till just after it slows down he log out and look at the logs).
It might not help but if you have something running at 100% or something off in the info could point at an issue.

I'm only thinking this as i had a gpu go strange on me a few years back, clocked it self down to 1/2 what it should have been, in the end i had to swap it out strip the drivers put it back and reinstall..

Post my be badly worded s i just got in from the pub and may have had a few..