With today's updated The Witcher 2 Linux beta is allowing the OpenGL worker thread to sleep when it can, optimized use of memory barriers, optimized fence handling, using ARB_texture_storage for texture uploads, and optimizations to the OpenGL state cache.
The beta notes also recommend the use of NVIDIA on Linux, which should come as little surprise to Linux gamers. "Most of these performance optimisations are best seen on nVidia hardware at the moment. Major performance improvements are expected in the near future through the work of AMD and Intel driver developer teams."
More details on this latest beta update, available via Steam on Linux, can be found via this GitHub page.