With native page transitions enabled, when a user navigates to a new URL within your app, the page will fade out, show a loading spinner, and then fade in once the new page is ready.
Specifically, the transition will start when a new non-AJAX HTTP request is made, and will fade in once the DOM readyState is "interactive". More info at readyState documentation at Mozilla.
Note: If your app feels slower than your mobile website, please first disable native page transitions, rebuild, and verify that it is now the same speed.
$( document ).ready() or similar so that it does not hold up the loading of your website, and so the native page transitions can finish.