This great plugin https://wordpress.org/plugins/query-monitor/ helped me solve a problem where only the admin side of my wordpress site was non responsive
– first it would give me a request timeout error.
– increased the PHP timeout limit
– admin pages load but it took 45 seconds or so to load a new page
– suspect problem is related to some plugin… but dont just want to randomly disable all plugins because the site has many and is an ecommerce site.
– install Query Monitor – instantly pinpoints the plugin causing the issue which turned out to be the Woocommerce Stamps.com plugin
– disabled woocommerce stamps.com plugin and everything is working again 🙂 – I didn’t actually use this plugin and don’t recommend it, the desktop version of stamps.com has so much better functionality that it is worth copying and pasting a few things..