One of the drawbacks of the Calendar in Own/Nextcloud was that it did not import calendar items into it - so if you wanted to show a third parties Google Calendar, for example, you needed to add that to all your clients.
Working on a Drupal site - no matter if your developing a module or doing minor theme tweaks - without turning the caching off is a pain. Here is a guide to disabling the cache for a Drupal 8 site. The good thing is, you only need one file to do it - so easy to turn it all back on again.
Just remember what you've done when trying to work out why the site is running slow ...