try turning 3d models on and off, changing any shaders/atmos, changing task priority in task manager to high, enabling GPU to use max performance if it's really that bad, changing the overall quality of the game visuals (which I assume youve already played with) enabling custom ghosts and disabling, these would be my first to trouble shoot.
I'm on a pretty dated system and these things seem to help a fair bit, running 60+fps with high speed ghosts, no custom ghost option enabled, dark floor shader etc.