This seems to be a really common issue with NextCloud. I'd say about 30% of installs seems to just..be slow? I've had this happen to me on a handful of installs, and i've had friends/collegues it's happened to.
I'm not aware of any "Fix" besides whiping your install(s) and trying again. Try not to use a backup if you can, as it can keep the slowness/lag across installs.
It's really annoying.
At that point it would be so painful it'd prompt users to switch to competing software.
The whole NextCloud suite seems to have this problem of too many offerings that don't get polished to completion.
Opencloud is the golang rewrite and is much faster.