I had reported this same type of problem myself.
In my case, I may even have to wait more than a minute after login to have the full display of the dashboard.
But I certainly made the mistake of posting in French. And I certainly didn’t get the most expert advice.
The problem is that people answer anything without reading, and they wanted to focus on the speed of my connection. It’s just stupid.
Especially with the nextcloud-desktopclient application, I have NO slowness problems. It is therefore not the server itself which is responsible nor my desktop.
The feeling I have is that nextcloud calls on one or more external resources which could, for example, not be accessible from my home (firewall or other) and which must timeout so that the process of constituting the page continues to load.
But bringing it up left the people who responded completely deaf.
8 months later, the problem is still the same for me. Nextcloud is therefore completely unusable.
But some people deny the existence of the problem which clearly affects a lot of people rather than taking it into account.