No idea unfortunately but I have heard that graphics issue (FPS, artifacts, etc) can sometimes be fixed by deleting a file in the WoW directory - I think the config.wtf file in the WTF folder - this resets all your graphics settings so you'll have to reconfigure the graphics settings the next time you're in WoW.

Aside from that if you use maximizer and PiP, when you flip between windows using PiP it can and will cause this symptom because sometimes it leaves the background WoW window running at full fps even though its in the background - once you have 2+ WoW windows running at 50 FPS it can cause an issue.