Let me start by saying this is only a guess based on what I've seen. It may well be related to the fact that I use 2 video cards {1 per monitor, non-crossfire}.

Check under WoW's Graphics options and set Monitor to Monitor 2. I did notice a huge drop in FPS if the game window was displaying on a monitor it wasn't assigned to in the ingame settings.

Before you do this, make note of your other graphics settings because it might reset some. Also, if you're using DX11, you might have to drop down to DX9 to get it to work at all. I'm pretty sure that's related to my setup because of the two video cards, though. WoW has an utter conniption fit if I try to use DX11 on my second card.