To be honest the game is reasonably optimised on a 6 core / 6 thread processor. When i was running the game on a 6 core i5 8400 the core running the main thread would sit around 95 to 100% and the other 5 cores would be up around 80%. There seems to be enough other work to keep 4 or 5 other threads fairly busy.
I have since upgraded to a 9900K which is much faster in game due to the 25% faster clocks. However of course now the main thread is still at 95 to 100% but the other 15 threads are all ticking over down around 20%.
This sort of explains why some new features (that can use the otherwise idle threads) have no effect on performance while other seemingly similar things (that rely on main thread) have a big performance hit.
The other interesting comment I read somewhere is the game makes use of AVX instructions, which if true are actually very efficient but cause some CPUs to generate a lot more heat.