As soon as I'm flying less than 100ft above the tree tops, my framerate is hit hard, down to 9fps. Makes hitting stick stirrers even harder to hit.
I usually fly with 40-50fps up in the sky. Framerate drops to 20 at 1k above ground level. 15 fps about 500' agl. I have my ground detail to a low setting. Maybe 1 mile. I know the slider is to the right of dead center for ground detail.
Basic Computer Specs:
Geforce4Ti4400 OC, 2.4Ghz Pentium, 1GB DDR400, Audigy2, 7200 8MB Cache 160GB Harddrive.