When I installed the game it automatically set the graphics settings to High, which caused a lot of lag as my PC is rather old. Go in an have a look what Preset your the graphics are set to - turning it down may help quite a bit. You can also try turning the graphics all the way down to Low, and then experimenting with turning individual settings up to see what effect that has on your frame rate. Things like anti-aliasing and ambient occlusion can be quite resource intensive, so turning them off/down can increase performance. Also, close other programs, such as web browsers or music players, if you have them running at the same time as you're playing.