Have you tried Nextcloud? Its slick right up until you get a few users that actually use all the core apps (contact syncing, automatic uploads, calendars, etc), then rendering the PHP login page jumps to taking 20 seconds of maxing out all CPU cores on the server.

Their IRC channel is no help in debugging. Disabling all apps doesn't help, looking at the php & syslog don't show any errors, and adding caching only saves a few seconds of rendering time.

Nextcloud has seriously lowered my standards for bad PHP projects, making most others look well written and decently documented/debuggable.