As absurd as it may sound, Apple has confirmed that older iPhones will get slower over time. And before you jump to conclusion, no it’s not planned obsolescence for the older device but a way to maintain the phone’s performance.
According to Apple, since batteries age over time and hold less charge, iPhones start throttling the CPU clockspeed to maintain a similar battery life and avoid unexpected shutdowns. However, Apple has implemented a feature for the iPhone 6, iPhone 6s and iPhone SE “to smooth out the instantaneous peaks only when needed to prevent the device from unexpectedly shutting down” and has also extended the same feature to iPhone 7 with iOS 11.2.
Apple’s reasoning makes sense but as The Verge points out, the company isn’t doing enough for consumers to educate about it and in fact, makes it difficult for customers to open and replace the battery themselves.