There could be numerous reasons. First thing to do is to be sure your system is clean and registry error free. Run CCleaner and do the Clean and Registry both. http://www.piriform.com/CCLEANER
The next thing I'd do is be sure old graphic drivers aren't conflicting. Uninstall your existing graphic driver, run Driversweeper in Safe Mode, and have it remove any Nvidia, AMD, ATI graphic driver remnants it finds. Reboot and install the latest driver for your card and O/S. http://phyxion.net/Downloads/
(don't fall for the ad in the red box)
Are you playing the legal or pirated version? the pirated version is known to have a bad crack which reduces frame rates significantly. Also the game is known to have issues with certain Radeon cards, and ATI have stated "The driver to fix current issues is slated for a sep 12 release."