This is one of the items that is tested in your Windows 8 App when you use the WACK.
If you are in Southern California, please email this post for more information on getting a token for your app by June 30, 2012!
First page must load in < 5 seconds.
First impressions are critical. Fast and fluid apps must have a good startup time. Aim for your application’s first page to be loaded (and functional) within 5 seconds from a cold start.
If your application takes longer, consider using an extended splash screen to keep the user aware of the progress. Once your app has loaded once, cache enough data or content to have a quick start-up that arrives at a functional screen promptly the next time it needs to load. If you load from cached data, refresh the data in the background to not block your loading experience.
Minimizing start-up time on HTML apps
Minimizing start-up time on XAML apps