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 (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 > 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.