Roon is multi-threaded. Much of it is for the sake of decoupling–database work, playback, user interface, networking. They all have different desired timing/response characteristics. Threads are a good way to separate them. In particular, we follow strict rules in the user interface thread (above and beyond what is conventional) to shift away as much work as possible and keep the UI responsive.
Because we use a handful of threads, Roon will naturally spread out a bit on a system with multiple cores. This isn’t a good, bad, or necessary thing…just what happens when schedulers do their thing.
I expect that most Roon machines look nearly idle most of the time, but our recommendations are there to make sure that they don’t fail when different kinds of loads are stacked up at once. When importing new content, performing audio analysis, applying library settings, and playing back in a few zones, things should still be stable.