Update: Apple has shared the following statement with TechCrunch confirming the functionality and its reasoning:
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.
Original story: Based on anecdotal observation, many iPhone users have long believed that older iPhones get slower over time. Generally, people have assumed that this is because of new features and additions in new versions of iOS that are better optimized for the latest phones.
But Reddit users, and Geekbench developer John Poole, have a compelling new theory, backed up by benchmarks: the iPhone may throttle performance to preserve battery life or avoid unexpected shutdowns as the battery degrades.