If you find that the app is lagging or freezing, there are a few quick steps that help fix that. First, try force closing the app and reopening it. Second, try deleting and reinstalling Simply Piano (just be sure to check which email you're logged into in the app under Menu > Settings > My Account to continue with your progress when you reinstall).
If the issue continues, let us know by tapping Menu > Settings > Send Us Feedback in the app. If you can include details about where in the app this is happening and what you're experiencing, that will help is look into it further to find a solution.