OpenGL speed changes from OS to OS. Here is the general order of backend speed (as I understand it:
Windows: Direct3D9
Linux: OpenGL
Windows: Direct3D11
Mac OS X: OpenGL
Windows: OpenGL
Linux OpenGL is basically on par with Windows D3D9. It's just Windows' crappy support and drivers do OpenGL that cause it to be slower. Mac OS X is slow on games in general.
Windows: Direct3D9
Linux: OpenGL
Windows: Direct3D11
Mac OS X: OpenGL
Windows: OpenGL
Linux OpenGL is basically on par with Windows D3D9. It's just Windows' crappy support and drivers do OpenGL that cause it to be slower. Mac OS X is slow on games in general.
Desktop:
OS: Windows 7 64 bit
CPU: Intel Core i5 3570K @ 4.4 GHz
GPU: EVGA NVIDIA GeForce GTX 550 Ti (Factory OC'ed)
RAM: 16 GB @ 1600 MHz
Dolphin Revision:3.0, 3.0-784, 3.5, latest 3.5
OS: Windows 7 64 bit
CPU: Intel Core i5 3570K @ 4.4 GHz
GPU: EVGA NVIDIA GeForce GTX 550 Ti (Factory OC'ed)
RAM: 16 GB @ 1600 MHz
Dolphin Revision:3.0, 3.0-784, 3.5, latest 3.5