You must have heard that Apple confirmed that they are slowing iPhones’ performance as their batteries lose capacity. And they lose capacity as they age which is normal for every rechargeable battery.
Apple says these (slowing down) measures are undertaken to extend the iPhone’s working time and improve its standby mode. It means that when iOS detect that your battery starts to lose the capacity the iPhone’s peak performance is being deliberately limited by using CPU throttling technology – decreasing processor’s frequency.