It seems that on Some GPU's at least the Nvidia drivers will put themselves into a failsafe mode following a Driver crash which massively downclocks the Graphics card.
The easiest way to reset it is to reboot.
I managed to cause it to happen after switching to single display mode after starting the game (Yes in hindsight that was a stupid thing to do
)
I've also caused it by alt-tabbing to much.
For instance my GT555m Should run at 590Mhz core (when not overclocked) however if it gets stuck in failsafe it runs at ~200mhz, which makes the difference between an enjoyable experience and a slideshow.
