I just checked google calendar it's under 3mb download for js (around 8mb uncompressed).. it's also a lot more responsive than nextcloud web. Even then, it's not necessarily the size, I think that's mostly a symptom of the larger issues likely at play.

There are a lot of requests made in general, these can be good, bad or indifferent depending on the actual connection channels and configuration with the server itself. The pieces are too disconnected from each other... the NextCloud org has 350 repositories on Github. I'm frankly surprised it's more than 30 or so... it's literally 10x what would be a larger expectation... I'd rather deal with a crazy mono-repo at that point.

gmail should be server sided, with as much JS as you want to use. Unless they moved away from the philosophy they started with GWT (Google Web Toolkit) for Gmail, and perhaps even Inbox (RIP)