One thing that could help with this is to use CDN for these static assets, while still having the Nextcloud hosted on your own.

We had a similar situation with some notebooks running in production, which were quite slow to load because it was loading a lot of JS files / WASM for the purposes of showing the UI. This was not part of our core logic, and using a CDN to load these, but still relying on private prod instance for business logic helped significantly.

I have a feeling this would be helpful here as well.