So, this isn't really a "fix", but it's some kind of workaround.
First, http://forums.steampowered.com/forums/showthread.php?t=2675769
This guy has made something great. Download that small file, run it and follow his instructions. CONGRATZ.
With this program I was able to run Skyrim, Hitman, Metro, Dead Space 2, Euro Truck Sim 2 (DON'T JUDGE ME), RAGE - pretty much everything in Bordered Window-mode without any problem.
And people who said it's a problem with WIndows 8, you are completely correct. Windows 8 basically caps the framerate of windows and programs running in the background - and running a fullscreen game will automatically do just that... Put everything else as background.
However, this is a fix until Microsoft eventually (hopefully) fixes this small screw-up. Happy gaming weekend, guys!