Common fixes
- Update PHP to a current version (see How to change PHP version). This alone often gives the biggest improvement.
- Remove unused plugins. Deactivating isn't enough; delete them.
- Find heavy plugins with Query Monitor, which shows how many database queries and how much time each plugin takes on admin pages.
- Increase the memory limit to 256M (see How to increase the PHP memory limit).
- Clean the database with a plugin such as WP-Optimize: remove old revisions, spam comments, expired transients.
- Limit revisions by adding
define( 'WP_POST_REVISIONS', 10 );to wp-config.php. - Reduce the Heartbeat API, which polls the server constantly while you're logged in; Heartbeat Control or your caching plugin can slow it down.
- Replace WP-Cron with a real cron job (see How to disable WP-Cron).
Check resource limits
If the admin is slow and you see 503/508 errors, your account may be hitting its limits; check Resource Usage in cPanel.
