Client Dropping Frames For No Apparent Reason

I've been playing WOW on the same client with the same addons on the same server and realm (Loarderan) since 2016. I've never before experienced framerate issues, my PC can handle games perfectly. For the last few days, however, I've been experiencing major framerate drops. For about 0.8 seconds i have 200+ FPS, then for the remaining 0.2 - FPS drops to 10-20. This occurs in-game, at the character select screen and even at the title screen, right as i have launched the game. It's 100% not my PC choking, because while running many apps in the background, I can play Deus Ex Mankind Divided at max graphics without ever dropping below 60 FPS (and that game is way more demanding thatn 3.3.5). Despite that fact, WOW still lags like heck. I've tried clearing cache, disabling and even deleting all addons, restarting the game and my PC multiple times, closing all other apps, so that all my CPU and RAM can go to the game - it still lags.

Any suggestions as to what i can do to fix this peculiar problem?

EDIT: I forgot to mention. When I play Vanilla, MOP, Classic and retail - this problem does not occur. So it is a WOTLK specific thing.

I have 100 Percent the same problem and cant fix it either.