I have an eight core FX processor, and find that Winblows 10 still handles more than four cores very badly, so as many of my processes as possible are micro managed. I assign affinities and priorities to everything. GTAV gives my system a work out, but it uses all four cores I assign to it and runs very well. So there are games out there that handle multiple cores well. Funnily enough, I've just gotten into making game videos. I still have a lot to learn abut that, but it's been fun.@Llewen - I also record and sometimes stream my gameplay with my multi-processor Alienware box using XSplit to HitBox.
I have basically "told" my vid/streaming software to use one processor, while UO/EC uses the primary. I forgot how I did it and because of that I have to be careful with Windows updates. It is also because of this that I do not like the EC too much, and play it minimally. Last time I seriously played the EC was.... uh.... dang, when I was still volunteering here at Stratics (couple years now I think).
To note, when I am live-streaming Diablo the system purrs like a loud kitten, but it does not overheat or have any game/system degradation. When I stream UO/CC, I rarely hear the fan. When I stream UO/EC, my system wants to scream so it is a very short lived session.