After a thread appeared on Reddit earlier this month saying that your older iPhone could get a speed boost if you replace the battery, some “Apple is evil” conspiracy theorists went into overdrive claiming Apple was slowing down old iPhones in order to get you to upgrade to new devices. As TechCrunch points out, this motive would be absurd. But Apple did confirm to the site that they do slow down older iPhones with battery issues in order to provide a better user experience:
Our goal is to deliver the best experience for customers, which includes overall performance and prolonging the life of their devices. Lithium-ion batteries become less capable of supplying peak current demands when in cold conditions, have a low battery charge or as they age over time, which can result in the device unexpectedly shutting down to protect its electronic components.
Last year we released a feature for iPhone 6, iPhone 6s and iPhone SE to smooth out the instantaneous peaks only when needed to prevent the device from unexpectedly shutting down during these conditions. We’ve now extended that feature to iPhone 7 with iOS 11.2, and plan to add support for other products in the future.