Some people report though that replacing the battery returns phone performance and CPU speed back to normal.
So it's true Apple intentionally slow down old iPhones. Proof: My iPhone 6 was bought 3years ago and recently got really slow. APP 'CPU DasherX' shows iPhone CPU is under clocked running at 600MHz. After a iPhone battery replacement. CPU speed resumed to factory setting 1400MHz. pic.twitter.com/pML3y0Jkp2