During Apple’s WWDC15, Apple announced that with Mac OS X El Capitan that it would have Metal integrated in to their newest OS. So far we haven’t seen any games come out that uses Metal yet. Until now. Well, in the coming month we’ll see it. One of Blizzard’s employees said in a WWDC 2015 thread:
“WoW is currently running with Metal. We will likely enable it before the end of the year.
We will do the same for SC2 and HotS in the next 6 months.
We will keep OpenGL for people with 10.9/10.10 or a computer older than 2012.”
Since World of Warcraft is already running with Metal these means more of Blizzards games are on the way to Metal. Which means more FPS (Frame Per Seconds) on older Macs.