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!
I would love to get an official response from Apple on this issue. Be sure to read the updates to this post, especially the second one.
UPDATE: Two things:
Via Bryan Lee on Twitter, if you are having issues with your iPhone 6s, Apple does have a battery replacement program for certain devices, manufactured in Sept or Oct 2015. Go to this page to check to see if your device is eligible.
As mentioned in the linked Reddit thread, there’s a free app, called CpuDasherX, you can use to check your clock speed, see if it’s running full speed. According to this comment thread, 1848 Mhz is the factory CPU speed for the iPhone 6s.
As noted in this tweet and some of the comments, the A9 does vary its frequency based on load. So a different frequency is not indicative of a battery issue. It’s possible this Reddit thread is a red herring. Again, would love an official Apple comment.
Take a look at this free battery life wear level tester. Might offer some insight.