"LunarG, on contract with Valve Software, discovered a critical shortcoming with the open-source Intel Linux graphics driver that was handicapping the performance. A special bit wasn't being set by the Linux driver but was by the Windows driver, which when enabled is increasing the Linux performance in many games by now ~20%+, which should allow for a much more competitive showing between Intel OpenGL performance on Windows vs. Linux. However, the patch setting this bit isn't public yet as apparently it's breaking video acceleration in certain cases."

-- source: http://games.slashdot.org/story/14/11/07/1237236

Cheers, Peter