CalDav unusably slow

Hi all,

I know that there’s been already quite a few threads created over the speed of Nextcloud. This issue, however, is specifically about CalDAV. The webinterface of the calendar isn’t hi-speed, but just fine. Calendar sync via DAVdroid is a massive pain, however. On both devices streaming web internet is fine, s

When I had two accounts synced that was ok. Now I have added my parents accounts (all shared with all) my phone is slow to sync and DAVdroid on my dad’s phone is stuck at ‘Configuration detection - request to server’ (until I reboot my RPi).

Set-up:
RPi 3, with HDD and connected to my home internet
Nextcloud version: 11.0.3 (stable)
Operating system: Raspbian 8 (Jessie)
Web server: Apache/2.4.25 (Raspbian)
PHP version: 7.0.16

Personally, I would expect loading a full UI would be slower than retrieving data only - but that doesn’t seem to be the case here. What might be the bottleneck in this sync process? Anything I can do to speed up CalDav sync? Any help much appreciated!