Although we are building a mobile application, it is common to run our app in a desktop browser or an emulator, especially while rapidly iterating on the code. Unless there is functionality that directly requires a physical device (the accelerometer for example), a lot of development work can be done well before the app ever sees a physical device.
While developing in this manner saves enormous time up front, it does mean that performance problems could often be overlooked until it is too late. Thus, it's a good idea to periodically run your app on physical devices that represent your target market in order to ensure that the performance remains acceptable.
Let's go over the typical differences you'll encounter between desktop browsers and emulators and physical devices.