Nvidia puts out very good kernel extensions (drivers) for OS X. Only issue is they only work for people with the 1st gen, pre-2013, Mac Pro.Couple reasons:
1. OSx, it's drivers, do not update quickly, nor are as strong performance wise as in Windows. Especially for gaming graphics.
AMD and nvidia in particular are frequently updating their windows drivers to provide the absolute BEST performance out of their cards in Windows. But the requirements in OSx have made this sort of quick update practice harder.
2. Apple itself does not really sell "gaming" computers. When you consider that Apple is roughly only 10% of the entire computing world. And of that small number, The computers they do sell mostly come with intel integrated graphics and IIRC, there's no current model with discreet GPU's.
Combining all of this, it make sense for them to focus most of their efforts on Windows based gaming. For now at least.
Apple really eneds to work on their gaming chops. games on the exact same hardware, between two different platforms can run anywhere from 10% to 50% slower in OSx.
An example recently I tested was World Of Warcraft (6.1 patch). fresh installs between the two. bootcamp for windows. OSx 10.10 for OSx.
at native resolution of the MBA, Windows was able to run WoW at native resolution, with 'medium' settings ran at 30-45fps.
in OSx, I had to run low settings, at a smaller resolution to maintain 25fps.
OSx is just not a great platform for gaming. its an unfortunate truth of the matter.
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what hardware?
Games run better on Windows because of DirectX. OpenGL just isn't as good.