The new iPhone 8, iPhone 8 Plus and iPhone X use a special “performance management system” from that in the old iPhone. This was mentioned by Apple in a special document describing the functions for improving the battery life.
According to the company, the new generation of the iPhone is equipped with advanced hardware and software shells that do not require user intervention in performance to extend the service life battery. The processor independently chooses such a ratio that the performance of the smartphone is maximum with minimal damage to the battery. So the owners of the new iPhone can not use the function to reduce the performance of smartphones in iOS 11.3.
Therefore, now the performance monitoring mode is available only on the iPhone 6, 6 Plus, 6s, 6s Plus, SE, 7 and 7 Plus. However, Apple does not deny that the batteries of the new iPhone are also subject to wear and tear, so in the future, the user will still have to replace the battery to preserve the device’s performance and battery life.
After installing iOS 11.3, all performance management functions will be automatically disabled for old iPhone. They will turn on themselves only if the device unexpectedly turns off due to a sharp decrease in performance.