Good to know it's not just me then, miumiaou! Is yours also slow when running at "Low" graphics quality?
Matt, I confirmed that it does still run at a low FPS, even after completely restarting the game after switching to Low. The slowness is noticeable right away from the menu, here's a screenshot with my Fraps framerate overlay in the upper left hand corner. It stays within a few FPS of that while in game, I can never get over 30:
http://i.imgur.com/Xnm2gA7.png
I tried, but I can't tell the difference in aliasing or anything between Low, Normal and Extreme graphics settings. Even when leaning in close and squinting really hard (a scientific method I came up with), the settings look identical to me.
I took some PNG screenshots through Fraps and tried zooming them way in to compare but again I can't tell the difference. Maybe you'll know what to look for. These were all taken after setting the graphics quality then completely exiting and restarting the game:
Low:
http://i.imgur.com/Z81GaaK.png
Normal:
http://i.imgur.com/7oLn0cs.png
Extreme:
http://i.imgur.com/eLJbUiv.png
Imgur claims it doesn't compress images as long as they're under 5MB, so those should all be original quality. Oh, and I forced off Vsync in the control panel so I could compare the framerate better, but it still has the same slowing issues and the other bug even with Vsync on. I normally run it with Vsync, and that's how it was when I first found the issue.
I will say that Extreme does give my video card a workout! It's usually running about 50-56C under load, and it got up to 60C with extreme. At Low and Normal quality the temperature drops right away though (about 45-ish and 50-ish respectively), so the three different settings are doing something.
EDIT: actually I'm wrong about part of that, I hadn't monitored the temps for a while, and other games get it quite a bit hotter (67C for Far Cry 3: Blood Dragon)