I run Opera on several computers. I do not copy my profile between the computers manually (sneakernet), but I do/have had Synchronization enabled. I have noticed that over time the browser gets slower and slower, to the point that recently it would freeze solid for 5-10 seconds, then work normally for 5 seconds, then freeze, repeat, CONSTANTLY. It become unusable.
I tried a lot of things to troubleshoot. I read about Flash permission issues, which helped a little. Then I deleted (moved away) all my profile data from C:\Users\Me\AppData\Roaming\Opera Software\Opera Stable and suddenly it became super-responsive again.
I played with it some more, deleting just one config file at a time. I noticed that the Sessions.db has grown to 350-550MB, depending on which machine we're talking about! When I delete that, Opera becomes responsive again. So my fix is currently to write down all the tabs I have open, delete all the Session.* files, start fresh, and open up all my tabs again. It immediately cures Opera... though I don't know how long until it builds up the huge DB again (currently at 40MB after 2 days).
I have Opera set to open from the previous session, where I left off. Those Session.db's have also been building since first upgrading after v12, and all the updates since. So I don't know if it's just that Opera never cleans up the DB because it's all the same session, or if it's an upgrade corruption, or whatever. (I'm sad to see the flat file that I could read, from <v12, go away.)
Anybody have any insight on this? Are there settings I can adjust to make the Session.db behave better? Is this just a bug? Am I using the browser in a way the developers just couldn't imagine, so I'm "breaking it"?