Players using iOS devices (iPhones and iPads) may experience some technical issues if they are not running on iOS 12 or higher. In order to maximize the application’s performance, we encourage all users to make sure they are using the latest version of iOS.

Not sure how to upgrade? Find out how from Apple: http://support.apple.com/kb/ht4623

Other steps you can take to improve performance: close other applications on your phone, and make sure you are using a strong internet connection.