Anyways, whenever there are 5 or more actors in my viewable area, whether they're behind a wall or not, framerate goes in the 10s. Any more than 5 it just gets worse, bottoming out at around 8, but that's the lowest it goes even if I add more actors. If I turn the camera away from the actors, framerate goes back to normal.
I'm running with 4x Wide Tent SuperSample AA, 16 AF, yet disabling those settings doesn't improve framerate at all.
CPU: Phenom Quad @ 2.94ghz
GPU: Radeon HD 5770 @ 960/1370
4GB DDR2 RAM @ 840 mhz
Edit: Running in Win XP 32bit (as opposed to Win 7 64bit) reduces the stuttering, but not by much. XP can handle about 8 actors before dropping to 10-16fps.
Edit2: It's probably also worth mentioning that dead actors do not cause the issue to happen, only living, active actors.