UPDATE:
I am getting over 500 fps with fog instead of the vector lines. So I'm sure this is the culprit. It might just be slightly slower on the AMD cards, but that's enough to go from 30 down to 5. I will experiment a bit more before updating again. I appreciate all the feedback!

If other Acknex games run fine, then it must be something wrong with my game, no?
I am going to upload another test version. But instead of vector lines, I will use standard fog settings. If you don't mind testing that, that would be great. That would indicate it has something to do with all the line draw commands...