A new Reddit post that gained traction this weekend is claiming that Apple is intentionally slowing down older iPhones in the iPhone 6/6s range with batteries that need to be replaced.
The original Reddit poster, TeckFire, said he saw significant speed improvements and improved benchmark scores in his iPhone 6s after taking in the iPhone for a battery replacement:
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!