Is there any possibility for getting the config file of yours. Because i am also facing the same issue of Nextcloud webpage is slow.
The Reason for asking the config file is like maybe i have misconfigured the Nextcloud Server our H/W specs may be different but tuning the Nextcloud is same for all users & The maximum load of my hardware hits not more than 20% of my allocated CPU,RAM & SSD Storage.
I have checked with page insights report from google of our Nextcloud webserver it is throwing me to clear the unused core-common.js
Most important is to configure a redis and possibly apcu cache. You could also check that the richdocumentscode app is not installed as it is known to cause slowdowns.
Lot of other apps could cause UI slow-down. In the past I hit an issue with external storage app…
I would recommend removing all apps you can - once the issue is solved enable them one-by-one to figure out which one is the problem.