I can say it seems some of the graphical options are BROKEN.
Possibly that's why TB noticed a change when he put everything in low, restarted the game, put it on high again, and restarted once more.
In specific, the SSAAx4 option in AA doesn't seem to work, at least with my amd card.
And more importantly, the fps limit option isn't working properly.
60 fps limit was giving me a very strange 30-50 fps performance. 90fps limit seems to limit it to 65 fps, and of course off it's giving me around 85 fps. So that's why the game is having a strangely bad performance for lots of people, and others were saying it was ok, some people were limiting the fps and others not.
Because with the theoretical (actually broken) 60 fps cap I was having a variable inferior framerate, I didn't think it had anything to do, but I was wrong.