I just checked and my second gmail login downloaded 500 KB before it became usable. Browser allows for caching of downloaded code.
> do not risk having to do it when being on a metered or spotty connection because your cache is not warm or the webmail got yet another update, which can happen at any time. You can apply Thunderbird updates when you are on a good connection.
I agree with your point. I think that it's possible to implement a separate update for web apps with explicit request, but it's rarely done and unusual for web.
> You can access your mails offline, you don't re-download them each time you open them. It's faster, even on a good connection.
Web browser allows for local storage and offline work which could be utilized for those features. You can enable offline mode in Gmail. I just checked, it works.