Along with testing the functionality of your app, it's also important to do performance testing on a variety of machines to make sure that your implementation is flexible enough to handle differences in processor speeds (for work on the UI thread) and GPU capabilities (especially affecting animations and rendering). Testing on different network speeds can also be helpful if you rely on online content–testing on a slow network, for example, can reveal the need to use progress controls in key places of your app (so the user doesn't think it's just hung). It can also help reveal places where you might implement strategies to avoid progress controls altogether, such as caching page content or switching between pages just with changes in visibility and not tearing down and rebuilding pages each time.
In any case, I wanted to provide some links to resources over a few posts. Here are a few topics in the docs for starters:
Performance (Windows Store Apps) – all languages
General Best Practices for Performance – all languages
More to come later on–this is probably enough for you to chew on at present!