This is not about stuttering or onscreen action causing a performance hit - this is about the framerate dropping to a specific number for no apparent reason.

Specs:
3900X
ASUS TUF X570
16GB C14 3600
5700 XT
2560x1440 @ 100Hz
Win10 x64 v2004
Games and OS on separate nvme drives.
All drivers up to date. CPU and GPU at stock; memory proven stable.

I just started playing again after a long absence and wanted to spend a little time in game before deciding to buy Greymoor. In general performance is substantially better than when I played before, but I am now encountering a strange issue where FPS drops from a solid 100 to 64 - and only 64, every time, before returning to normal after ~15 seconds. I don't know if it's because I alt-tab and switch between borderless and fullscreen that returns the FPS to normal or if it does it on its own.

Hardware monitoring (HWINFO64) reports no change in utilization or temperatures, and it happens any time in any area which suggests to me that it is not a client-side rendering issue.

Is anyone else experiencing this? That it reverts specifically to 64FPS is both interesting and aggravating.

edit: I just remembered that a large part of the performance improvement initiative focuses on optimizing workloads across multiple cores. I wonder if disabling the SMTs (AKA threads on a real CPU) would help at all. It's not like any game or software I use is taking advantage of the 12 actual cores, let alone the 12 SMTs. I will test it.

edit 2: Neither disabling SMTs, completely reinstalling GPU and chipset drivers, or reinstalling the game made any difference. It really is disappointing.