The slug is rather pinicky when it comes to that.
I’ve had good experiences with nice levels though. I run firefly at nice level -5, and wizd at -10.
So during other cpu-intensive activities those apps are still responsive, if a little slower.
I’ve cron-jobbed a rescan at 5am every day, just after my podcasts are fetched. Which is normally enough for my slug to pick up new music, too.