words from the WP8 SDK documents:
How to test the retail version of your app for Windows Phone
Before you release your app in the Marketplace, we recommend that you build it as a native image, to test its performance on a device as users will experience it.
When you build your app in Visual Studio, the code is not compiled into a native image, but into a machine-independent Common Intermediate Language (CIL) binary file. (CIL was formerly known as Microsoft Intermediate Language, or MSIL.) This CIL file is what you submit to the Marketplace when you're ready to sell your app. At that time, the binary file is converted from CIL to Machine Dependent Intermediate Language, or MDIL. Finally, when the user downloads your app to a device, the MDIL file is linked to produce a native image. You can repeat these steps in your development environment by changing certain build options in Visual Studio.
The functionality of your app is not affected by the native code generation. However the native image typically starts and runs faster.
To build and test your app as a native image
In Visual Studio, on the Standard toolbar, select Device as the deployment target. You cannot test native code generation on onthe emulator.
In the Solution Configurations drop-down list, select Release to create a Release build. You cannot generate native code for a Debug build.
On the Tools menu, select Options to open the Options dialog box. In the navigation pane, select Debugging.
In the list of debugging options, make the following changes:
Deselect the Suppress JIT optimization option.
Select the Enable Just My Code option.
Click OK to close the Options dialog box.
Build or rebuild your app, and test its performance and responsiveness.