I have a feeling you are using the Demo or V1 at best, though you did not specify. I was playing around this morning and put 20 characters in a room about the size of a basketball court with me to see what they would do. I'm using V1.05 RC2 and all 20 were active, shooting, and aggressively perusing me at once. The FPS started at 17 and increased as I killed them off, but that's pretty much to be expected.
I have no idea what your computer specs are either. Mine is more than a year old.
AMD Athlon 64 3000+ (1.79Ghz)
1GB PC-3200 DDR RAM
Radeon X600 256MB PCI-E Video
Best.
I'm sorry, my answers are limited. You must ask the right question.