Apple says slower performance of older iPhones is intentional
 

You're not alone if you've noticed a slowdown in the performance of your older iPhone. The thing is, it may be more related to your battery than the phone itself. After a post on Reddit and a followup by benchmarking software Geekbench's founder, App...

https://www.engadget.com/2017/12/20/...s-intentional/