The Windows Phone Application Analysis Tool
Not only is performance important in ensuring that your app provides an enjoyable experience for your users, but it is also important in a stricter sense: for meeting the certification requirements of the Windows Phone Marketplace. Marketplace certification includes a number of performance related criteria that your app must adhere to. The requirements are as follows:
If an application performs an operation that causes the device to appear to be unresponsive for more than 3 seconds, such as downloading data over a network connection, the app must display a visual progress or busy indicator.
An app ...