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
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.