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.