Valve is working on porting their games on the Linux platform, and they recently tested Left 4 Dead 2 (L4D2) on Linux (OpenGL) and Windows 7 (Direct3D) on the same hardware (Intel Core i7 3930k + GeForce GTX 680 + 32GB RAM).
On Windows 7, L4D2 ran at around 270 FPS while on Linux, the same game ran at 315 FPS. More than 45 FPS or in terms of milliseconds, the OpenGL render path is 0.5ms faster than the Direct3D render path (milliseconds are less impressive than FPS!). The interesting thing is that the D3D version of their game engine has been polished and optimized for years while the OpenGL version is still a baby! The boost in performance comes from the underlying efficiency of the Linux kernel and OpenGL (NVIDIA drivers). But what are the performances with other drivers like AMD or Intel ones?
Valve has also tested their OpenGL engine on Windows 7 and the game ran faster than with the D3D engine:
Interestingly, in the process of working with hardware vendors we also sped up the OpenGL implementation on Windows. Left 4 Dead 2 is now running at 303.4 FPS with that configuration.
It’s a great for the OpenGL community!
Source: Faster Zombies! (Valve blog)