The Wicher 2's Linux port is basically using a wrapper layer for its Linux compatibility rather than being a full-blown, native Linux port. The Witcher's wrapper layer isn't Wine but uses eON by Virtual Prgramming. About eON, "We have an established and robust technology, known as eON, which was created from the ground up for gaming. It is a fully native implementation of essential Windows technologies (such as Direct3D), developed with support from our close relationships with Fortune 500 hardware and software technology companies. Our technology allows us to be flexible with all games, providing a fully-optimised and native runtime for each game, giving the player the best available experience under Mac and Linux. It is also possible to deliver a Windows-based game to these platforms without source code: an important factor where developer licensing may be restricted."
This eON wrapper layer though reportedly is allowing for better performance than running the Windows version of The Witcher 2 under Wine, but still it's not without its limitations. Many Linux gamers have been venting their frustration about poor performance, this eON wrapper reportedly limits the game engine to being single-threaded, and there's ongoing issues with GPU drivers. Right now for The Witcher 2 on Linux, there's only dominant success stories heard from those with the binary NVIDIA Linux graphics driver on GeForce hardware.
There's people sharing their frustration about The Witcher 2 Linux performance and this eON layer via SteamCommunity.com and 80+ comments within our forums.
Hopefully the Linux GPU driver issues will be quickly worked out and that there's room for more optimizations for The Witcher 2 Linux performance, but the game regardless is a beast and mandates rather high-end hardware. If you're looking to upgrade your Linux gaming rig for this title and the many other upcoming Linux AAA games, read our thousands of Linux hardware reviews.