Looking at the UI I can see why it's fast - everything is set up in a way to maximize performance if you're willing to put in the work.

Which is to say, simple text, simple lines, simple shadows - all of which, so long as you take the time and draw smartly using Core Graphics, is well within the capabilities of any iOS device. Combine with a few dashes of smart UI caching and the whole thing will scream.

iOS UI optimization is a topic I wish more devs knew about. So many relatively simplistic apps are dog slow because no one has bothered to do even the most modest amount of optimization.