My iPhone 6S has been very slow these past few weeks, and even after updating multiple times, it was still slow. Couldn’t figure out why, but just thought that iOS 11 was still awful to me. Then I used my brother’s iPhone 6 Plus and his was... faster than mine? This is when I knew something was wrong. So, I did some research, and decided to replace my battery. Wear level was somewhere around 20% on my old battery. I did a Geekbench score, and found I was getting 1466 Single and 2512 Multi. This did not change wether I had low power mode on or off. After changing my battery, I did another test to check if it was just a placebo. Nope. 2526 Single and 4456 Multi. From what I can tell, Apple slows down phones when their battery gets too low, so you can still have a full days charge. This also means your phone might be very slow for no discernible reason. Check your Geekbench scores and see what you get if your phone is still slow!
TL;DR Apple slows down phones with low capacity batteries, replacing it makes them full speed again. Check Geekbench Scores.
Edit: some more information from all of these comments, very informative. Anyway, apparently no one with a 5S has been affected here so far. Not sure why. I also found out a bit more about *why* this happens. Let me explain: Many people might remember that iPhone 6S battery fiasco, which for many, was fixed with iOS 10.2.1, and that seemed to be the end of it. Apparently, the way it did this is by dynamically changing the maximum clock speed relative to the voltage that the battery is outputting, so that your phone can’t draw too much power and shut down. I also don’t know for certain, but I believe that the iPhone 7 and up are not affected by this nearly as much, since the A10 and A11 chips have separate “low power” cores that apparently run at around 1/5th of the power of the main cores, and will use those as often as possible, and only kick in the fast cores when necessary, meaning you should only see slowdowns for more intensive tasks on the 7 and up, since only one set of cores can be running at once. Geekbench should still let you know though, regardless. Also, some apps might be able to tell you your current clock speed, and if this is true, you can use that instead to find out exactly how much your phone is downclocked, if at all. I will put my old battery back in and verify this when I can.
Edit 2: Some information on replacing the battery: you have 3 options.
1. Go to Apple. They will fix it for you with a 1 year warranty, and you are guaranteed it will work. It is $79 to get a new battery from them.
2. Go to a reputable third party repair shop near you, some examples are Experimac or Batteries + Bulbs, they can do it for $30-40 usually, and under and hour if you come in at the right time. You should get a warranty from these shops if they’re any good.
3. Go on iFixit and do it yourself. Provided you have the right tools, (and you MUST have the right tools) you can do it yourself, for a first time repair, in 30 minutes or less. Be very careful, look up guides, and be prepared. Batteries can be found on Amazon, eBay, or on other specific sites such as MobileDefenders that sell phone parts, usually from $9-20 depending on who you buy it from.
Edit 3: This is not changed when plugging into power, it still results in a slow phone if your battery has a low enough voltage, and therefore your cpu has a lower clock speed.
Edit 4: thank you to u/Adolf-Intel for [his comment](https://reddit.com/r/iphone/comments/7inu45/_/dr0miiv/?context=1) on CpuDasherX, a free app from the App store that I just used with my old battery to verify it, and it does indeed show a reduced clock speed. Use this to test if your phone has been downclocked, and by how much. you should be able to look up what the original clock speed is for your device and compare it to what the result is in the app. (Scroll down on the default app page and it should be near the top of the screen when scrolled down to the bottom)
Edit 5: CPUdasher X is no longer free, and is now $0.99. If anyone knows an alternative that gives actual clock speeds, that would be very much appreciated. Also, I’ve seen reports that the A9 chip has a variable frequency, and changes depending on the load, and while that may be true, I think enough of us have seen the evidence to show that it gets locked down and won’t go back up when you have too much wear on your battery.
Edit 6: I looked into making my own app for testing clock speed, as I am a developer, but can’t find enough information on how to figure that out, and don’t have the knowledge required to get the clock speed. Hopefully someone can help me with this, or make this app themselves.
Edit 7: thank you to u/Kalloud for [their comment](https://www.geekbench.com/blog/2017/12/iphone-performance-and-battery-age/)because Geekbench just gave us some solid proof of this issue.
Edit 8: Apple has confirmed this. This was their statement:
> 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.
As for why this happens, many people have pointed to it being a battery issue, however, no matter what brand of battery you buy, the result is the same. The original Apple batteries on many 6S models were crap, yes, but it was later fixed, and again, 3rd party batteries fare no better. I think this means it must be a hardware problem, with the processors drawing more current than a battery can provide. As to why this doesn’t happen with Qualcomm chips? I think it is because Qualcomm chips are less powerful than iPhone chips, at least with the A9 and up, within the same time period. As in, when the A9 was released, it was more powerful than anything Qualcomm had on the table. This has continued to be the case with the A10 and A11, and while Qualcomm has caught up since then, they have had more time to deliver the same performance. To add to that, the multiple low power cores will fare better than apples design of few high powered cores, meaning again, more power saving, at the cost of per-core performance. This is speculation, but there is some evidence to back this up.