This might help some people:

http://forums.steampowered.com/forums/showthread.php?t=1328904

It helped a little bit, but not much. When I’m playing the game, I get about 40 to 60 fps on max settings. But when theirs like 10 NPC’s in the area it goes all to hell and drops to 8 to 10 fps! No matter if I have everything set to the lowest settings, its still the same frame rate :cryvaultboy: ! Anyone else have any fix!?

Really, if anyone on the forum has calivoyance, that ability isn't known to me. I certainly am not. If and when a complete problem report, including the standard PC Gaming Computer hardware component summary, and a routine software environmental description is provided, then perhaps someone will be able to offer a suggestion. Sorry to have nothing else for you now.

http://www.gamesas.com/index.php?/topic/1123068-what-that-dxdiag-thing-includes/