As the battery of your iPhone degrades, Apple throttles the speed of your iPhone. What was once just a hunch from people who feel annoyed that their old phone “feels slow” now has hard data and an Apple statement to back it up.

A Redditor noted earlier this month that his phone speed increased significantly after he replaced his iPhone 6S battery, and had benchmarks to prove it. A followup post by John Poole of GeekBench (a benchmarks company) found the same. iFixit teardown engineer Jeff Suovanen performed similar tests with iFixit employees’ phones and shared the data with Motherboard.

Suovanen found that iPhone 6S devices that still had their original batteries (they are about two years old now) had benchmark scores that were up to 57 percent lower than the GeekBench average. Replacing the battery instantly improved the benchmark scores drastically; he saw 70 percent swings in benchmark performance after swapping the old battery for a new one.

"Everyone came back a day later and said, 'Wow, it works so much faster,'" Suovanen told me on a phone call.

Phones that were performing far below the GeekBench average suddenly began performing above it after he swapped in a new battery.

“The takeaway is that the original batteries were causing a lot of CPU throttling, and replacing the batteries seems to have completely cured that,” Suovanen said. “We’re familiar with the fact that older batteries lose capacity, but we don’t expect it to cause a big hit on performance. This was an eye-opener for me.”

Apple told TechCrunch that it throttles iPhone performance to prevent the phone from being shut down if a performance spike draws too much power. Lithium-ion batteries lose capacity over time, so such a system is fine and makes sense. What doesn’t make sense and is indefensible is that Apple has not been forthright about this behavior. The statement Apple gave TechCrunch is impenetrable to the average user:

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.

What makes it worse is that Apple does not make it easy to replace the battery yourself, discourages third party repair, and doesn’t have the first party repair infrastructure to handle large numbers of in-store battery swaps, especially in states that don’t have lots of Apple Stores.

“It’s a reasonable thing to do but it’s sketchy to do it without disclosure,” Suovanen said. “What people inevitably think is, ‘My phone is slow, I need to replace it.’ And that causes a lot of perfectly good phones to get replaced.”

I just called my nearest third party repair shop; they do iPhone 6S battery replacements for $39 and it takes 15 minutes. Apple charges $79 for this service and you need an appointment (surely many people simply decide to buy a new phone they don’t need).

To be clear: You can make your old iPhone faster if you replace the battery. You can do this yourself if you’re brave, or take it to a third-party shop if you want it handled by a professional.