It's not as simple as that. DX may be a wrapper for what is the basis of OpenGL, but it uses a different approach and optimizes rendering for effects and quality.
The outcome is, that with OpenGL you got to program many things manually that you get for free in DX. You can see the effect in IL2, as under DX the textures are smoother and look more real, though the engine was optimized for OpenGL!
So depending on the application you can be faster with OpenGL, leaving out some of the routines of DX, while on other applications and especially when working for games and such renders, it's often more practicable to use DX, as you will more easily see, what you'll get in the end.
OpenGL has just the advantage, that it is widely opensourced, while you make yourself a "slave" to M$ when using DX.
|