So i was noticing low FPS today, I was hovering around 40.
When i hit my windows key or targeted a browser window on my second monitor the FPS jumped up to 150+. Upon re-engaging my WoW client, the fps would drop back to 40.
As a frame of reference, I had backround/foreground fps limiters disabled, and tried (after noticing the issue) to enable them, but max them both out.
If I play in Windowed mode, i get a consistent 140-170 FPS. Upon re-enabling windowed fullscreen mode, however, the FPS drops back to 40.