What card do you have? And what settings are you running the game on?
Problem solved its not my cpu's fault ... after i installed latest nvidia 8800 gt driver somehow physyX in mirror's edge got bugged and lagged my pc... but if i shut it off it doesnt lags... i better send a mail 2 nvidia
if you have downloaded Mirror's Edge is gone to hell with PhysX ... or you need to download the latest patch for Mirror's Edge The latest Patch fixes the PhysX problem crashing or low FPS in the game
i've updated the game..and the game still lags in some areas. so the patch doesn't fix much.