What exactly is the resource-intensive request here? Loading an E-mail, or list of E-mails? I don't see why that should be any more resource-intensive than any other CRUD app.
Sure, gmail optimizes for this heavily so it's fast, but it's still one of the most intensive things you can do for an app like that, so reducing the amount of times you need to do that is a huge win for any webmail. If you've ever used a webmail client that's essentially just an IMAP client for a regular IMAP account, you'll note that if you open a large inbox or folder it's WAY slower than trying to view an individual message, most times, for what are obvious reasons of you just think of a mailbox as a database of email and the operations that need to happen on that database (which it is).
If clicking on an individual message is a new page, that's fine, but if going back to the main view is another full IMAP inbox query, that's incredibly resource intensive compared to having it cached in the client already (even if the second request is cached in the server, it's still far more wasteful than not having to request it again).