Not paying attention.

1. Indiscriminate use of packages when a few lines of code would do.

2. Loading everything on every page.

3. Poor bundling strategy, if any.

4. No minification step.

5. Polyfilling for long dead, obsolete browsers

6. Having multiple libraries that accomplish the same thing

7. Using tools and then not doing any optimization at all (like using React and not enabling React Runtime)

Arguably things like an email client and file storage are apps and not pages so a SPA isn't unreasonable. The thing is, you don't end up with this much code by being diligent and following best practices. You get here by being lazy or uninformed.

I think, some of the issues here is that first nextcloud tries to be compatible with any managed / mutualized hosting.

They also treat every "module"/"apps" whatever you call it, as completely distinct spa without proving much of a sdk/framework. Which mean each app, add is own deps, manage is own build, etc...

Also don't forget that app can even be a part of a screen not the whole thing