Constant CPU usage while idle (not playing music) after stopping playback

This is what I see when I stop playback, with Shortwave installed from flathub:

I'm going to venture a guess: you're using GTK spinner widgets and you don't stop them from spinning when they are hidden? (same as https://github.com/GeopJr/Tuba/issues/145 and GNOME/gnome-calendar#971 (closed))...

If so it should be stopped as soon as the spinner is not needed anymore, it will reduce CPU usage during and after playback.