Apple has added a Battery Health section to some iPhone models in iOS 11.3. The performance management feature was created in response to Apple's CPU throttling controversy. The new section in the Settings app allows users to view their maximum battery capacity and disable the performance management feature.
All rechargeable batteries, including iPhone batteries, become less effective as they age. Older batteries have a lower charging capacity which can limit the amount of usage time between charges, and it can cause unexpected device shutdowns. The "performance management" tool throttles your CPU when peak battery performance can't be obtained by your device.