Quoted in saying:
Reddit users have noticed that Apple appears to be slowing down old iPhones that have low-capacity batteries. While many iPhone users have experienced perceived slowdowns due to iOS updates over the years, it appears that there’s now proof Apple is throttling processor speeds when a battery capacity deteriorates over time.
Also folks also discovered after replacing their iPhones battery it went back to normal.
This what Apple has to say:
Here’s Apple’s full explanation, provided to Business Insider by a representative:
“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.”