So I got this game during the sell a few days ago. When I first bought it I installed it and started it up to make sure it worked and messed around with the settings to get what looked good to me and what ran well on my system. I ended up with:
Filtering: 2x
AA: disabled
Lighting: High
Texture: High
Shadow: High
Grass: High
Decals: High
Depth: no
Visibility: Near
Animals: High
Resolution: 1280x720
I played for a short bit and everything ran smooth. However last night I was playing and all of a sudden the whole game slowdown to unplayable FPS, I restarted my computer and went back into the game and it was running at the same speed. I decided to turn my computer off and tried it again this morning and it was still running at the same FPS. I turned the settings down to:
Filtering: 1x
AA: 0
Lighting: Medium
Texture: Medium
Shadow: Medium
Grass: Low
Decals: Medium
Depth: no
Visibility: Near
Animals: Low
Resolution: 1280x720
At those settings it still ran at the same speed, no improvement what so ever. I can play games like Crysis on all High settings with little slowdown ever. I'm baffled that I can't figure out why this game won't run. Any help would be appreciated.
Specs:
i5 2.4 GHz processor
ATI Radeon mobility 5470 HD
4GBs Ram.

Check you saves. Probably you have too many of them.
TW does not overwrite saves - every a save is made, a new file is created. The number can got quickly overboard if you like to quicksave a lot.

It stays the same when you start a new game? Maybe the problems are area-oriented.

Yes it stays low when I start a new game. That's why it's so weird, It worked fine before but now whenever I start a new game or load the one save I have it's to slow to play. I'm running the GOG version through the Steam overlay so I tried to run it without Steam over it but that didn't help, and I also tried to delete the one save I had but it didn't work either.
Edit: Just tried to play the game with all the settings on the lowest possible, still the same FPS :/

Any automatic upgrades in the meantime? Background processes running?

Normally I would think about drivers' issue, but since it worked and stopped overnight that's unlikely.

In the Task Manager it says I have 69 processes running which is the usual for me. There have been no automatic upgrades to my OS. Also during this time I've tried running other games and they all run perfectly fine, it's just Witcher.

You didn't post your OS. Are you running the game as admin? And did you install it as admin too?

Sorry about that, Windows 7 64-bit and yes I'm running as admin and installed as admin as well.

Just re-installed it, still no luck. This makes no sense.

Thanks for all the help Thiev. I ended up going to the CCC for my ATi card and reset everything to factory settings and that solved the issue. Once again thanks for the help though anyways.