I Have just downloaded witcher 2 from gog and ı start playing but game is very very very slow even with minimum graphics...
Heres my Pc System
4GB DDR3 ram
intel İ5-4200U 1,6ghz(2,6 turbo) processor
AMD HD8750M 2GB DDR3 GPU
You can try lower the game resolution (e.g. 1366*768 ) but I'm not sure if it helps since you're saying that the game is very very very slow.
Also I suppose you're playing on notebook so make sure that it's not overheating and the game is using the AMD graphic card, not Intel.
I've played Dragon age:Origins with everything maxed out on 30fps and i have the same fps in Witcher 2 with everything on minimum so my guess is that your AMD card is not working properly.
The last thing I can recommend you is update graphic driver (http://support.amd.com/en-us/download).