it is pointless to talk in theory without actually trying things out, besides the app doesn't have to do only email, in fact it already has p2p support for real-time that can be used for now inside the https://webxdc.org/ mini-apps, and might be used for calls in the future, or just use WebRTC for calls, one of the mini-apps in the store "Live Chat" already has typing indicators btw. The app already has Jitsi Meet invitations integration.
Chatting in Delta Chat with chatmail feels just as fast as WhatsApp, Telegram etc. sometimes even faster depending on the chatmail server you use
I know, but I don't want to self-host it, I want to use it completely offline as an app on the phone just as google translator app, also LibreTranslate can't translate in real time using the phone camera like Google Lens