Games on windows 8 in bootcamp lag even on lowest graphics
I had something similar happen to me recently. Suddenly, game performance went down pretty badly, not only on Windows, but on OSX as well.

Reseting the SMC seems to have corrected the issue.

Here are instructions on how to do it: http://support.apple.com/kb/HT3964

Hope it fixes your problem, if you haven't yet found a solution.