Thanks for the help, I will try and see what happens.

Also installed query monitor plugin to see what it sees

the slow query is

SELECT post_id, meta_key, meta_value
FROM wp_postmeta
WHERE post_id IN

update_meta_cache() calls it, I wonder if I can prevent it from running....

It's something to do with your database.  Not sure what it is, but that's in *every* installation of wordpress by default.  So, if it was a problem because of the number of posts, there would be more hue and cry about it.

Check the indices on that table, and repair them- especially if they are on post_id.  You might also try to run mysqlcheck on all of the tables, i.e.

mysqlcheck -u root -p --check --all-databases

or...

mysqlcheck -u root -p --auto-repair --check --all-databases

if you're going to trust it to check.  Or you can just run the auto-repair after the check.