Can I set the nice level for Nix build processes?

I have found these two settings: NixOS Search - Loading...

  nix.daemonNiceLevel = 19;
  nix.daemonIONiceLevel = 7;

But that doesn’t actually make the builds themselves use these nice values from what I’ve observed.
It appears to affect only the nix-daemon process itself.

Is there a way to also set nice for all build processes Nix daemon spawns?

if the nix daemon is spawning the build, in theory the sub process should inherit the parents nice value… unless its spawning the builder in such a way that it’s not honouring this.

it also may/may not - be effected by autogroup … I’ll look into this later when i get a min to try it out.

autogroup is enabled by default on nix/OS …so it maybe a contributing factor?

Maybe I was checking this incorrectly but when I looked at htop the only process with nice value of 19 was nix-daemon.