The usual reason for WordPress admin panel slowness is broken DNS.
Most Distros now use systemd, which in turn uses systemd-resolved, which is badly broken. Meaning this code fails to correctly cache DNS lookups.
The result… Each admin page visit requires looking up all remote links, like external site API calls. Since DNS caching is broken + calls to down sites have a timeout of many seconds or minutes… admin pages slow to a crawl.
The fix I use for all my hosting clients is strip systemd-resolved out of all LXD containers (use to host client sites + APIs) + replace with dnsmasq.
Using dnsmasq tends to fix many slow admin page problems.
Unfortunately most hosting companies are clueless/useless, so if you admin your own servers, strip out systemd-resolved. If not, it’s unlikely you’ll find a hosting company able or willing to do this.
You can tell if this is your problem by installing the Query Monitor plugin + then run with SAVEQUERIES enabled in wp-config.php + look for changes to red (slow) in the Query Monitor status bar.
When the bar turns red + clicking on it shows many HTTP requests in red, then it’s very likely you’re getting hit by the systemd-resolved broken code problem.