It seems that deleting the APEX_ClothingGPU_x64.dll allows the game to run a bit faster in some cases with some nvidia cards. Give it a try, there's a backup file (i'd recommend just moving it to the desktop, running the game and just moving it back if it doesn't help.

INSTRUCTIONS: Go to your Witcher game directory then into \bin\x64 and delete the APEX_ClothingGPU_x64.dll. I've uploaded a backup of the file in case you lost it, the file version that comes with the game is 1.3.0.7

My fps on ultra settings (without hairworks) goes from 38 to 43 average and CPU usage goes from 25% to 30% average. I have a Mobile i7 920XM overclocked to 3.6GHz and a Geforce 770M overclocked. Your mileage may vary!

This tweak came from the description of Real Graphical Suite - I would recommend checking out the mod and it's description which contains great info on improving your visuals and performance with this game.