Theres quite a large thread on this on the Steam forums at the moment - http://forums.steampowered.com/forums/showthread.php?t=2513477
Running in windowed mode seems to work for a few minutes, but will eventually start slowing down just like it does in fullscreen. (tested on max gfx, max VRAM+normal VRAM)
Major slowdown during screen transitions (when it wipes the screen to black for a second)
Very noticable when theres a lot of objects on the screen or having menus and character convo windows open. For example if I start running then hold up to stop quickly, the smoke effect on the ground causes my framerate fullscreen to drop from 60 to around 20.)
Quoting what I posted on the Steam for you guys -
Just popping in to say that disabling my second monitor did NOT resolve the issue, sadly.
However running the game in windowed mode does solve the problem, but its rather small.
Intel Core 2 Quad 2.33GHz (Q8200)
Nvidia GTX 480
8GB DDR2 1066MHz
Windows 7 Prof x64
Monitors - 2 (1600x900[60Hz], 1024x786[75Hz])
EDIT - Running in windowed worked for a little while, but after a few minutes of play it slows down to the same sort of speed as when it was running on full screen. Memory usage is just going up and up as well, could be a memory leak as stated earlier.
EDIT-EDIT - Just tried it on another PC in the house, exact same issue.
Intel Core 2 Duo 3GHz (E8400)
Radeon HD 2400 Pro (Old, but definetely good enough to run a game like this at max.)
Windows 7 Home x64
Monitors - 1 (1440x900[60Hz])
I don't really want to disable my other monitor...
C'mon matey, its not like its hard to turn it back on again. Seems theres a 50/50 chance of this working for you, so you might as well try it.