I found that
itunescloudd was sucking every available CPU cycle. I had to sign out of iCloud completely and then reboot in order to get it to stop. Signing back into iCloud started it right back up again. Even a "reset all settings" didn't help - only signing out of iCloud.
It seems this thing is far from ready for prime time (GM soon? Apple had better rethink that!
). While the UI has been fairly stable including most apps, I've had nothing but problems with anything to do with iCloud, iTunes, the Music app, and syncing. Many of these (especially sync issues) aren't new problems to iOS 7; seems like more than just beta issues, but poor software design in general...