Now that we've hopefully convinced you to verify performance on physical devices, how does one identify that performance is becoming a problem? There are a couple ways:
Physical testing involves an actual interaction with your app on the device. As you test your app, you may encounter visual stutters, delays, lags, or worse. It's also important to test on as many different classes of devices as possible, since each kind and model often have very different performances. This can quickly become expensive, but this can be mediated to some degree with a large beta test with lots of human testers.
Profiling looks at several different metrics to identify where potential problems may be. Memory is the most obvious metric; you can record your app's memory utilization during your interactions in order to identify any potential memory issues. You can determine long-running processes in your app that may be causing visual stutters as well. Profiling inevitably introduces its own performance...