When you run a wine emulator or something to run DirectX on Linux, sure games run poorly. When you run a Linux compatible game with OpenGL games run just fine on Linux. The problem isn't performance the problem is Devs support Linux and OpenGL. The good news is engines like Unreal support both and quite a lot of games run on Unreal. It only takes a few leaders to get the rest to follow. Building games for Linux isn't that hard thanks to engines like Unreal who do all the heavy lifting for developers. It really comes down to the market not demanding it, hopefully SteamOS changes that. The other thing to consider is that SteamOS isn't a full Linux distro, it's a watered down version of Linux specifically designed to run games. Think of Android OS which is the same kind of thing only designs specifically to run on phones.