You said it's feature rich, therefore has lower performance over slow connections.

This is not true. Mobile app performance over slow connections is determined by app/server architecture, payload efficiency, caching strategy, etc.

There are many examples of feature rich apps performing well on slow connections, and many simple apps performing poorly over a normal connection.

On my android phone, the mattermost app does something weird to my keyboard - it replaces the right-most 1/3 of the spacebar with a key with a '->|' symbol on it that closes the keyboard when pressed. I've never seen any other app do this, and it's (of course) really annoying because it means when I try to hit the spacebar the keyboard closes instead.

That's just one of a couple issues I have with the app (push notifications not really working, the chat taking 15 seconds to reload everything upon switching back to the app, etc) that have basically made it useless to me.