Matthew Panzarino has details:
As that battery ages, iOS will check its responsiveness and effectiveness actively. At a point when it becomes unable to give the processor all of the power it needs to hit a peak of power, the requests will be spread out over a few cycles.
Remember, benchmarks, which are artificial tests of a system’s performance levels, will look like peaks and valleys to the system, which will then trigger this effect. In other words, you’re always going to be triggering this when you run a benchmark, but you definitely will not always trigger this effect when you’re using your iPhone like normal.
Apple will continue to add this smoothing to more devices over time to avoid shutdown issues, freezing and other problems.
I can see why Apple limits peak performance on devices with degraded batteries, but most people will just hear “Apple slows down older phones.”
I’m not sure there’s a way around that, but Apple should alert a user when their iPhone crosses the battery threshold and the phone begins to operates in this mode, giving them information about replacing their battery.