There is no reason why 20 people casting autoattack has to be run in series instead of parallel. tho admittedly thats a bad example cause it's server side. but you get the point. combat and draw calls can be multi threaded with a proper engine, which would help so much.
i pray to god they can find a way to get dx12 into this game a couple years from now, but i fear it just won't be doable, just like they could only get a few dx10/11 features to work.