There are a number of techniques for increasing the performance, and perceived performance, of applications built with the Xamarin platform and Zebble.
Your app's pages should load very quickly. If they have contents that come from a Web Service (or Web API) call then lazy load such content. Before your page is shown, it should not make any Web Service calls in general.
Set android:hardwareAccelerated="true" in AndroidManifest.xml file.