As your app approaches completion, you may wish to fine-tune it for memory usage, speed, and other real-time behavior. Xcode provides a sophisticated and powerful utility application, Instruments, that lets you collect profiling data on your app as it runs. The graphical display and detailed data provided by Instruments may give you the clues you need to optimize your app.
You can use Instruments on the Simulator or the device. The device is where you’ll do your ultimate testing, and certain instruments (such as Core Animation) are available only for the device; on the other hand, certain other instruments (such as Zombies) are available only in the Simulator.
To get started with Instruments, set the desired destination in the Scheme pop-up menu in the project window toolbar, and choose Product → Profile. Your app builds using the Profile action for your scheme; by default, this uses the Release build configuration, which is probably what you want. Instruments launches; if your scheme’s Instrument pop-up menu for the Profile action is set to Ask on Launch, Instruments presents a dialog where you choose a trace template. With Instruments running, you should interact with your app like a user; Instruments will record its statistics. Once Instruments is running, it can be further customized to profile the kind of data that particularly interests you, and you can save the structure of the Instruments window as a custom template.
Use of Instruments is an advanced topic and beyond ...