The third public Beta of Witcher 2 has been released on Steam.
If it will improve the modest performance on Linux has yet to be tested.
To get always the newst beta of Witcher 2, opt-in in your steam-client!

Changes according to https://github.com/KillaW0lf04/The-Witcher-2-Issues

11 Aug 2014 14:34 BST

Public Beta 3 - buildID 352258

This contains the following: