I've been having the exact same issue. After 2.1 my game was fine and ran as normal (about 60-70fps) on highest graphical settings. However, after the hotfix the problem described above has been incredibly prevalent. I'll get a few seconds of alright (50 fps, maybe?) performance and it'll just drop to the point others are jittering across the screen (10 fps). It's especially jarring in dungeons and it's making trying to PvE at all a great pain.

I should note turning settings down fixed nothing.