If you've noticed your older iPhone slowing down, the problem might be tied to the battery.
John Poole, founder of Primate Labs, the company behind the Geekbench processor benchmarking software, says in a blog post that iPhone 6 and iPhone 7 phones have a problem that is "widespread, and will only get worse as phones (and their batteries) continue to age."
He found that as the batteries get older and lose capacity, their processors slow down, decreasing performance. Poole explains that users expect their phones to perform the same regardless of how old the battery is, but his tests indicate that isn't the case.
He says the problem is due, in part, to a change in Apple's iOS operating system, perhaps as part of a "fix" for an issue with some iPhones shutting down suddenly.
This fix will also cause users to think, "my phone is slow so I should replace it" not, "my phone is slow so I should replace its battery". This will likely feed into the "planned obsolecense" narritive. [sic]
The planned obsolescence in Poole's final line above refers to the idea that tech companies purposely slow down or otherwise hamper the performance of their products to get users to buy a new one.
Apple did not immediately respond to a request for comment.