My framerates are cut in half or worse when the trees show, and that's with a 6800GT and A64 3200. I've had to turn those sliders waaay down, which is too bad. The trees really do look nice but going from 70 fps (lcd refresh rate) to 25 at low alt when the trees fill more of the view than a tiny square of trees beneath the plane kinda sucks. Maybe there's an issue when the trees start approaching the haze or horizon distance? I dunno, but sometimes I'll look in one direction and see nothing obviously different in that direction from any other direction, but the framerates will drop 20 fps in that direction. Trees seem to be the only common thread - the lower I am and the more of the screen filled by the trees, the worse the framerate is. But sometimes I'll get a framerate hit in just one direction even without seeing many trees, so it's a bit of a mystery to me.
1280x1024, 512 textures, sliders turned down enough so the trees go the heck away at typical dogfight altitudes.
