Yes, Apple Is Slowing Down Your Old iPhone. Here's Why—and How to Fix It
“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.”
Rather than have the device shut off in unfavorable conditions—older batteries that can’t handle peak draw, cold temperatures, etc.—iOS updates to iPhone 6, iPhone 6s, iPhone SE, and iPhone 7 cause the phone to purposely throttle CPU clock speeds.
As AppleInsider points out, lithium batteries deplete over time. How much Apple throttles your processor depends on the wear and tear on your battery.
Reddit users point out that replacing an iPhone battery ($79 with Apple) leads to “returned performance and CPU clock speeds back to normal,” according to The Verge. This may not work for everyone, though, as battery wear and tear varies. Replacing an iPhone can run anywhere from $349 – $1,149, depending on the model and the amount of storage.