I know there are issues around hardware utilisation existing in the game for some people and that a patch is in beta to address that for steam users. In the meantime, I've had a play with the visual settings and found a significant increase in the frame rate when the object detail is turned down from 100% to 0%. originally I was running about 15 fps average, now I'm hitting around 25 - 30 fps.

The effect gameplay wise isn't particularly noticeable aside from some pop-in on objects and grass, it's worth it for the smoother experience in my opinion. Hope this helps anyone having frame rate issues! :)

FYI I'm running a nvidia geforce 280 gtx, 8gb ddr3 Ram and an i5 760 processor.