It isn't uncommon for older iPhones to feel a tad sluggish after a few years. New iOS updates do bring new features as well as make things more efficient but, as we've recently learned, they also throttle performance. Apple says there's a good reason for this.
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.
Apple is saying that iOS throttles performance on older devices for a reason, not to give users the impression that their older iPhones are slowing down or to push them to consider upgrading to newer models. It seems batteries are the limiting factor here and, since iPhone's aren't known for being easy to open up and get their batteries replaced, some customers are not to thrilled by the news.
Apple's decision to slow down performance is a remedy, a fix for premature shutdowns last year for iPhone 6, 6s and SE by smoothing out CPU demand when a battery is older, cold, or just low on juice.
Unfortunately, this isn't transparent to users, who may think something is up with their iPhone and would not know any better. It seems that replacing an older iPhone's ageing battery (a way cheaper option than replacing the whole phone) can restore performance, that is until that new battery starts to degrade in the future.
Source: The Verge