Apple largely develops their own GPU drivers for Mac OS X in-house and last year they overhauled their OS X OpenGL stack for NVIDIA and AMD graphics processors.
Source: Phoronix
While the Mesa / Gallium3D graphics drivers on Linux leave a lot to be desired, in terms of features, supporting the latest OpenGL specifications, and performance compared to the multi-platform proprietary Linux and Windows drivers from NVIDIA and AMD, the Mac drivers aren't too much better.
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Source: Phoronix
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