Apple this week admitted its iOS software slows down the performance of older iPhones. The company says iOS does this to counteract problems found in aging lithium ion-batteries. When a battery gets older, it doesn't hold a charge as well and can unexpectedly shut down if it's put under too much stress. Apple's software prevents that from happening by slowing performance.

https://www.cnet.com/news/apple-slows-down-older-iphone-battery-issues/

https://www.cnet.com/news/apple-is-slowing-down-older-iphones-batteries-faq/#ftag=CAD590a51e

"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."