Maybe there's low disk space.
Nope, I've got the same issue on my 1020 with 12.8GB free. Also did a hard reset before upgrading to 10512...so it can't be either of those things.
Maybe that's the key, is to backup and NOT restore. Maybe there's some crap being loaded with the restore that's not optimized or compatible yet. And yes, my phone got super hot.I started to have the same problem a week or so ago. After a few days, I just did a reset and did NOT restore anything. It's been running great since.
I noticed that it happened with weak or disconnected wifi as well. Does yours get really hot too?
Did you hard reset after upgrading?
Yes this build is unstable. MS has a lot of work to do this month....