Been having the same problem myself. Playing for over a week full-screen on dual monitors with no issues, as recently as yesterday. I'm not aware of any patches to the game, the most recent auto-update by Windows was over 2 days ago, and I certainly didn't change any settings or update any drivers manually, but suddenly the game can't handle full-screen on my primary monitor at more than 5-10 FPS.
I've noticed that during loading times the second monitor seems fine, but as soon as any sort of user input is available (even something as minor as the login text screen) it flicks off and game performance tanks.