When you create an app for windows phone, Visual Studio adds frame rate counters to visualize rate properties. To disable those, we need to open App.xaml.cs and need to set a variable to false as seen below:

// Set it to for not to display the current frame rate counters.
Application.Current.Host.Settings.EnableFrameRateCounter = false;

All required information about these counters can be found at the address:
http://msdn.microsoft.com/en-us/library/windowsphone/develop/gg588380(v=vs.105).aspx