All I can do is point out the official minimum requirements for Mac.
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NVIDIA® GeForce® GT 640 or AMD Radeon™ R9 M290X or better - Metal capable GPU
The GT 640 is a desktop GPU from 2012. The Radeon R9 M290X (Neptune XT) is a mobile GPU from 2014. Hence Desktop Macs with 2012 hardware should still run HotS but laptops need 2014 hardware or newer.
D3 and SC2 (HotS engine) use completely different shader designs. It is possible that the SC2 shader designs require some Metal API features to work efficiently that are only offered on sufficiently modern hardware while D3 did not.
You could try campaigning Blizzard if there is a way for them to lower the minimum requirements of HotS as far as Metal API compliance goes without tanking the graphic quality or performance for modern Mac OS users.
For reference, supported Mac players reported huge gains in frame rate during the beta of this patch. For example from the game barely running at low with decent frame rate to playing on Ultra hitting the refresh rate. How much of this low performance before was caused by lack of proper OpenGL support (compared with Windows) one may never know.