A few quick steps should help fix any lagging or freezing in the app:
Force close the app and reopen it
Delete and reinstall Simply Piano (before you do this, just be sure to check which email you're logged into in the app under Menu > Settings > My Account to ensure you continue with your progress when you reinstall).
If the issue continues, let us know by tapping Menu > Settings > Have a Question? in the app. If you can include details about where in the app this is happening and what you're experiencing, that will help us look into it further to find a solution.
Have more questions? You can talk to us through the app. Tap on Menu > Settings > Have a Question? :)