The problem could be when something new is written to the db. A possible error in db? A slow db?

I posted above the ways to find out.

Your problem apparently is happening when logged in and posting/saving. So we can't test.

So it could be something with the database. Also when doing all that press f12 and check your browser's console for errors.

Also enable debug.log, and check.

This kind of problems come from plugins which affect WordPress loopback function.

Please read this support topic and try to find the plugin which crashes your website: https://wpforo.com/community/how-to-and-troubleshooting-2/slow-when-posting-a-new-topic/paged/2/#post-37642