BabaJ

If under the drop down menu that you normaly choose a view and/or Karma CPU/XPU Storm etc., there is a side menu 'Render Settings'.
If you select 'Viewport Settings'....then bring up the Display Options:solaris panel with spacebar +D.

Then under the Render Tab select Karma CPU from drop down 'Show options for'.

Under another tab 'Basic' there is an 'IPR Reserve Threads' that might help you.

This is strange to me, I always thought IPR 'stuff' was part of the Mantra frame generation but not Solaris(usd).

But...it might be worth giving it a try as it is under Karm CPU options and not greyed out.

It's for where ever you have set on the render rop - 'Render to Disk', 'Render to MPlay' or 'Render to Disk in Background'.

I don't think there is such control for the viewer rendering as it is part of the application(Houdini).
As such resources like CPU is controlled/delegated by the OS.

Under the tab 'Output' of the usdrender rop there is a parameter 'Maximum Threads' you can tick on.

Read the tool tip so you can set with a positive or negative number according how you want it specified.