Well no wonder! I have a better CPU and GPU and can't do that. Turn physx off and see if that brings your frames up
Even at lowest settings the framerate is poor. The thing is cpu and gpu are not loaded properly.
Borderlands 2 and Mafia 2 are the only 2 games that have unexplanable lags on my pc. Mafia seems to have some game engine problems, it lags even on GTX Titan.