Read original post by Mike Battista at The Windows Phone Developer Blog
The fourth of the four principles I mentioned in Optimizing Apps for Lower Cost Devices is Respond to User Input.
A responsive UI is a basic expectation that users have of apps. Failing to respond to user input can frustrate users and can ultimately drive them away to other apps that are more responsive.
The basic guidance here is to keep as much activity off of the UI thread as possible until absolutely necessary. The UI thread is what processes user input for you, so any code you execute on that thread will interfere with this processing. Unless you're updating elements of your UI, you generally should not be executing code on the UI thread.