Slow back-end in live site

Is it happening to anyone? I have +300 sites. Front-end works pretty well. Back-end instead is really really slow.

My testing sites require 1-2 seconds (so, Pressbooks is ok and my server too) but my live site requires 10-15 seconds.

I did clean old post, transients, optimization of database… Test with the plugins deactivated, custom theme deactivated …

any suggestion?