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.
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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 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)
