https://getkirby.com/changelog/kirby-2-3-0 says:
After an update from Kirby 2.x.x to Kirby 2.3.0 ALL thumbs are build new!
This is until all thumbs have been build by visiting all webpages with thumbs…
Hint:
I have done the update locally, then done the building of the thumbs there, upload the thumbs to the webserver and then done the update on the webserver.
Yes but it wasn’t the issue since I regenerate them before testing.
How did you regenerate them?
All thumbs changed the filename and the path from the old Kirby version to the new…
I purged the thumb folder and visit all pages one by one.
I had your same issue with the panel taking long to save pages. In my case it was because of some hooks happening on page update (Panel page update ( Hook )). It seems to be a bug. I applied this fix and it worked good again https://github.com/getkirby/kirby/pull/452/files