A Reddit post over the weekend has drawn a flurry of interest after an iPhone 6s owner reported that a battery replacement significantly increased the device's performance running iOS 11. The ensuing discussion thread, also picked up by readers in the MacRumors forum, has led to speculation that Apple intentionally slows down older phones to retain a full day's charge if the battery has degraded over time.
This is an interesting case. The testing methodology would be interesting to see here, since you can also replicate worse results by running the same test multiple times in a row, since the A-series processors don't hold up to a sustained load like a desktop CPU.
But if this is indeed legit, I honestly don't think it's a bad decision by Apple. If your phone has a worn out battery and the system has to choose whether to make your phone last longer or run faster, I think making it run a little slower so that it doesn't die is the right call. It's essentially doing what Low Power Mode does already, just without your need to toggle it on or off.