For all the downvoters: I changed my iCloud password two days ago. Messages stopped syncing so I did the same thing I always have to do which is disable the account in Messages, restart the app and re-enable the account. Now for the last 12 hours the app is using 100% CPU and is still not caught up with all messages.

But yeah I'm sure that's my fault.

After all syncing messages from iCloud is a highly CPU intensive operation. It's a miracle we can do it at all.