Speed-Up Slow WordPress Dashboard

My WordPress front-end behaves acceptably, yet the admin dashboard crawls and occasionally throws a “website under heavy load” message. I have not added any new plugins or themes, and a caching plugin is already active, so the usual quick fixes have been exhausted. What I need now is a deeper optimisation pass: pinpoint the queries or processes clogging the back-end, clean or re-index the database if required, fine-tune the current caching layer, and adjust server-side settings (PHP, MySQL, HTTP/2, object cache, etc.) until the dashboard opens swiftly and reliably. Please run benchmark tests before and after your work so I can see tangible improvements in load time and resource usage. Access to the hosting panel, WP-CLI and any monitoring tool you recommend can be arranged immediately.