John Poole, the Geekbench processor benchmarking software founder, said in a blog post that processors in iPhones slow down and decrease in performance as batteries age and lose capacity. And Apple has shared an official statement in response to the report, and admitted slowing older iPhones to prevent battery issues:

Our goal is to deliver the best experience for customers, which includes overall performance and prolonging the life of their devices. Lithium-ion batteries become less capable of supplying peak current demands when in cold conditions, have a low battery charge or as they age over time, which can result in the device unexpectedly shutting down to protect its electronic components. 

Last year we released a feature for iPhone 6, iPhone 6s and iPhone SE to smooth out the instantaneous peaks only when needed to prevent the device from unexpectedly shutting down during these conditions. We’ve now extended that feature to iPhone 7 with iOS 11.2, and plan to add support for other products in the future.

Read also: How to fix iPhone 7/7 Plus battery draining fast and overheating issues?