[bartbutler]

1. ProtonMail implements the OpenPGP standard and is fully interoperable with other OpenPGP email systems. 2. The web app is a single page application so it does not reload on every request.

That said, you are correct that the web app is not appropriate when the threat model includes ProtonMail itself (though you can run the web app locally and thus sidestep the problem). The native clients are better suited in that case.