this happens only when battery is already unable to power the device at peak consume. With this feature the iPhone can be used anyway without turning off. this happens on any moderns devices since CPU have peak power consumption much higher than idle consume. Users should, like with laptops, change battery in 3-5 years with correct usage.
iOS don't let the device to use peak power, so you can see a quite difference in benchmark but much more less in real life usage. This is intended to help users, simple as that, if you think otherwise you are just plain wrong.