But Mongo DB has no joins?

Backend rendering- user won't see anything until you all your queries finished

Most people would tell you that a database lacking joins is not appropriate as your datastore. They will tel you that you need joins and relational DB when you display your UI. That would make sense for a typical web app, the one where you render everything on the server. You need to render a page ideally as fast as possible, because the user won't see anything until you all your queries finished.

SPA- user sees the page loading with the first chunk of data

For SPA type of web apps, this makes little sense. Typically you want to query small chunks of data, not one big chunk. That way your app can feel much more responsive and your front end can be written much simpler than if you have one big piece of data coming back. Keep in mind, that round trips to the DB are expensive and structure your data to be as flat as possible and you should be ok.