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.