You can try my Lemmy app, Arctic. It’s a pure swift based iOS app, written with performance, and content presentation in mind. It’s packed full of features, and new ones are being added constantly.
You can read more about Arctic over at !arctic@lemmy.world
The mobile theme on lemmy.world is Voyager. You can see it linked on the lemmy.world sidebar. Voyager is just a web app, so it can be hosted by instances as a mobile UI.