Your tests are very interesting how the OS changes CPU render speed. Didn't expect it to change at all. Maybe I should do some test renders to compare High Sierra and Mojave. I have Mojave on a 2013 Mac Pro, and I'm finding the interface laggy compared to High Sierra. Like moving around the viewport, there is a small lag which is annoying. Also dark mode doesn't work in open / save windows so kind of feels half done to me. So yeah not really into Mojave and now wishing I hadn't formatted and clean installed it onto my Mac.
I can also confirm Mojave does not work well with Cinema4D R18, its laggy when moving around viewport. This is on a 2013 Mac Pro. On my older 2009 Mac Pro with High Sierra Cinema4D R18 doesn't have these laggy issues. Darn it I shouldn't have updated, hard to know until you try it though. I wanted dark mode, but dark mode doesn't even work in open / save windows! This could be another reason to move over to PC land?