When running Windows 8 I got 75-85 FPS , installed 8.1 and now I get max of 15 FPS. I tried reinstalling 8 and updating to 8.1 ,installing ONLY NEW DRIVERS and the game itself , and still get the lousy framerates. I restored a backup of Windows 8 , and I get the 75-85 FPS again .
Running a AMD Phenom Quadcore 3.2 Ghz processor
8 Gb ram
Nvidia 770GTX with 2Gb video ram
256 GB SSD boot drive with 75 GB free
1Tb raid array for games using 2 500GB SATA 3 drives in raid 0.

Any idea why I would get reasonable framerates in Windows 8 , and such horrible framerates in 8.1 ? Like I said , using all of the latest drivers , updated 8.1 etc. . I even set everything to lowest settings in graphics setting in game , still can't get over 15 FPS.
In Windows 8 , with the 75 FPS , everything was maxed out .