Here are some troubleshooting things to try:
- Restart the computer (As you've tired, of course)
- Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this:
Start up your Mac in safe mode - Apple Support
- Make a new user and see if the same problem recurs with it. This is a bit harder, because a new user can't access a different user's stuff. So you'll need to connect the new user to your Apple account and you would need to move your Library out of your own user's folder up to the general "Users" folder.
- Rebuild your Photos Library, again--close Photos and re-launch by option-command-clicking the app icon. Since you're using iCloud Photos, you might want to turn off iCloud, perhaps turning off wifi, itself. When you reconnect, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.
The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.
Let us know what happens…
