A couple of weeks ago, Apple had admitted that they were intentionally slowing down older iPhones with a bad battery. That had created a furore as it was perceived by some as a way to get existing users to upgrade to their latest iPhone.
According to Apple, the move was necessary to prevent sudden iPhone shutdowns and to squeeze as much battery life from a faulty battery. Those affected can regain their full performance by swapping their iPhone battery for a new one. As a gesture of goodwill, Apple has offered discounted battery replacements for the iPhone 6 and above.