Agree on what you said, but not true at on this point. DirectX does things better than OpenGL, but also OpenGL does many things better than DirectX.
The bigger impact, but that is a subjective point only: DirectX has a lot of resources for developers (new and old) on its side to help create nice, optimized code., while OpenGL, despite free and wildly used, is on the lacking side in comparison, and DirectX has more developer tools, let alone more powerful, especially that DirectX was built for debugging as it was designed for gaming. OpenGL wasn't really... OpenGL was this Silicon Graphics internal thing what wans't complete, which they only released to the public as the company was shutting its doors. Even though it has been ages since OpenGL was released, and some tools are there to help OpenGL developers debug things and get better understanding on the background, it is still lacking compared to DirectX. This is because the foundation of OpenGL needed to be changed, and hasn't been done enough to really do that. So more tools for developers, can help shine DirectX over OpenGL.
But the real performance increase is really Windows and drivers. Windows as its design, since Windows 95, games running on full screen with DirectX gets to run closer to the hardware. Something that OSX and Linux based OS doesn't do, for them, a game full screen or not, is like any other program. This doesn't make it a miracle solutions, but it helps give that performance advantage edge, and also ensure more consistent performance. And then drivers. The simple fact that Windows is the most used OS, hardware manufactures puts all their energy on delivering quality and highly optimized drivers on Windows.
Mac = metal potato
PC = Toaster
Toaster > metal potato
Mac is for work and boils down everything to its basic form (even though I find windows easier) and Pc just is good all round.
The thing about macs is that it slows down way too often when using Google, my laptop that cost 700 dollars can run google with 10 tabs no probs (Could just be me...)