I'm not sure what is going on but it's a constant issue. If I alt-tab to my second screen (whether I'm in Window mode, Fullscreen window mode, or Fullscreen mode) and alt tab back into the game, my framerate is stuck at a lower framerate.

As an example, I will be running at 50+ FPS. However if I alt tab for something and tab back, my FPS is stuck at 20 FPS. Only way that I've found to reset is to go into the graphics setting and change the default video setting from Desktop standard (which is what I normally have it set it at) to Desktop High, and then back to Destop standard.

Pastebin link to my DxDiag report
http://pastebin.com/0B58RDYv