Preventing unexpected shutdowns
About a year ago in iOS 10.2.1, we delivered a software update that improves power management during peak workloads to avoid unexpected shutdowns on iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, and iPhone SE. With the update, iOS dynamically manages the maximum performance of some system components when needed to prevent a shutdown. While these changes may go unnoticed, in some cases users may experience longer launch times for apps and other reductions in performance.
Customer response to iOS 10.2.1 was positive, as it successfully reduced the occurrence of unexpected shutdowns. We recently extended the same support for iPhone 7 and iPhone 7 Plus in iOS 11.2.
Of course, when a chemically aged battery is replaced with a new one, iPhone performance returns to normal when operated in standard conditions.