Looked into the User.cfg thing a bit more.
for Intel 265k - and presumably other CPUs where you don't want to use all the cores.
Turn console on in settings, restart game.
Open console (the ` key next to 1)
Run command: Render.DrawScreenInfo 1
This will display the number of threads the game will create.
For the 265k it defaults to 18 threads. It has identified 20 cores and left 2 free.
In the User.cfg you can add these two lines, setting X to however many threads you want the game to use.
The other lines from User.cfg examples spammed all over the internet aren't necessary.
Thread.ProcessorCount X
Thread.MaxProcessorCount X
I tried 16, 12, 8 - and the performance was 140fps (my cap) for all of them.
Using 8 instead of the default 18 brings the cpu usage down from 100% (room=inferno) to ~50% (room=normal).
So that seems like the way to go.
I still need to test numbers between 8 and 12 to see if there's a perfect number where the game just barely becomes GPU limited.
That's for another day.