(Linux 64bit)

Whenever asset forge looses focus (window minimised or click another window) the cpu usage shoots up from almost nothing to 100% on one core.

Check the configuration of Asset Forge and either lower the visuals, or enable v-sync if it isn't enabled.

I disabled all graphic "enhancements" and ensured that vsync was set and even with everything turned on while the window has the focus the CPU usage is very low

regardless of various combinations of settings, as soon as the window looses focus it pegs a core at 100%

Which Linux distro are you using? That way I can check myself.

I'm using Void Linux (gcc libc not musl) 64 bit

 [chris@chris-N150CU ~]$ glxinfo | grep "renderer string"
 OpenGL renderer string: Mesa Intel(R) UHD Graphics (CML GT2)

 [chris@chris-N150CU ~]$ cat /proc/cpuinfo | grep "model name" -m1
 model name : Intel(R) Core(TM) i7-10510U CPU @ 1.80GHz

I did boot into an old version of ubuntu and that pegs a core at 100% unless the window is maximised, but its a year or more since I booted that horror show...

Best guess I'd say Unity and gfx driver combo...? but even Kerbal Space Project doesn't peg a core, so not sure.