I had a similar problem with slow performance on new or editing post / pages and I tracked the problem back to a plug in called Keyword-Statistics. It’s an auto generator of keywords etc from your post and a lot of my posts had a lot of information in them so Keyword-Statistics was working overtime with the database and generally being a pain in the arse.
Once it was uninstalled , hey presto everything was back to normal.
Other than that can’t help if the problem is else where
