Benchmarking software company Futuremark has put the iPhone 5s, iPhone 6, iPhone 6s, and iPhone 7 through its tests and found that each phone maintains its performance even after a new iPhone is released.
Futuremark's tests reveal that processing chips inside iPhones as far back as the 2013 iPhone 5s perform just as well while running Apple's latest iOS 11 as they did when they ran iOS 9, which was released in 2015.
Futuremark found the same results for the iPhone 6, iPhone 6s, and iPhone 7.
Some conspire that Apple intentionally slows down older iPhone performance with every new iPhone release, a tactic known as "planned obsolescence," in order to encourage iPhone owners with older devices to upgrade to a new device. However, there is no data or proof that acknowledges that Apple is engaging in any planned obsolescence.
Instead, an older iPhone's performance is more likely impacted by new versions of iOS and the ensuing app updates, which are usually released in conjunction with a new iPhone. New versions of iOS, as well as apps designed to support the iOS update, often contain features that are designed to work fluidly with the hardware inside new iPhone models. With this in mind, it's understandable why older iPhones can slow down when new iPhones roll out. New versions of iOS and apps that roll out after a newer iPhone's release may simply not be optimized to work as well on processors inside older iPhone models.
Futuremark also suggests that the perception that your older iPhone is slowing down is compacted by a "psychological effect," where "knowing that there is a new and improved model available" makes your older iPhone seem inferior.
While it's true that older devices generally get slower over time, fairly recent models like the iPhone 6s are still a fantastic performers. My iPhone 6s Plus is still performing as well as I want it to, despite the fact that it's a two-year-old device.
