It appears that enabling Ambient Occlusion leads to a very, very large frame rate drop. I had everything on high/enabled (except VSync), and I enabled ambient occlusion, without moving, looking at a relatively open area, and my frames dropped from 155-170 fps to 57-59 fps.
That definitely doesn’t seem right, and it adds a noticeable amount of input lag (I have a 120hz monitor, so I’ve gotten used to about ~5-8ms, and 16ms makes mouse movement kind of floaty).
I’ve got an i7 @ 4.2ghz, and a gtx 580, so it seems odd that ambient occlusion would result in such a large frame rate…