Later updates pushed to the S4, such as Android 5.1.1, were notorious for introducing performance slowdowns and battery drain on the aging hardware of the S4. Many users find that the 5.0.1 version offered the best balance of features and battery life. If you are currently on a buggy later version, "downgrading" (or re-flashing) 5.0.1 can breathe new life into the phone.