I've never used nextcloud, but I always imagined that the point is you can run services but then plug in any calendar app etc. You don't have to be running nextclouds calendar, I thought. Did I misundestand how it works?
Their calendar plugin provides CalDAV, so you could just use your local calendar app that syncs with the server over that protocol.
If dav works best for you, you're using it right.
I would assume that the people for whom a slow web based calendar is a problem (among other slow things on the web interface) are people who want to be using it if it performed well.
They wouldn't just make a bad slow web interface on purpose to enlighten people as to how bad web interfaces are, as a complicated way of pushing them toward integrated apps.