Hi there.
Like the title says i have many CPU cores with their threads… (AMD TR 3960X). though WoW only uses 1 thread @ 100% witch can only feed my GPU for 40% of it’s capacity… (Nvidia 2080 Super). So i know that if the game would support 2 or more cores/threads my FPS would be a lot better and the game would run much smoother. It’s 2020 soon and the engine really needs an upgrade blizzard! The Multi threaded 8.1 update was a good start (unfortunately only for GPU’s) 
Blizzard techs know this obviously but it’s not gonna happen soon.
They would have to make a whole new engine, because you cannot just add more multithreading to such an ancient engine (maybe 18 years or older).
Lots of stuff is so deeply integrated in that old tech that they just can’t change it anymore.
It is actually not exactly a given that software running on more threads/cores is automatically notably performing better - that honestly largely depends on what the software is meant to do, and how it does that.
Specifically for games where a lot of processes are essentially flows of A—>B—>C (with B not being able to actually occur before A has been resolved and C not being an option before A & B are taken care of) multiple threads would bring often only rather small performance gains (if any at all).
Given this is not actually a technical problem (nothing is broken, game works as intended) either and the topic really falls under the “Suggestion” category I’m going to close this thread here. Have a good one y’all. 