"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." https://www.apple.com/iphone-battery-and-performance/

Unexpected shutdowns vs. longer app launch times and other reductions in performance. a compromise.