Well, check in your config.wtf file for the following:
SET gxRefresh "3000/1000"
If such a line exists, change it to:
SET gxRefresh "60/1"
Also, running the game in a fullscreen window can cause fps drops, double check that the following is set:
SET gxWindow "0"
3rd, it appears from your picture that your desktop (or at least the taskbar) is visible over the top of WoW, if so, the graphics card/CPU may be treating WoW as a background app and not giving it a full work load.