An update to iOS will let you stop Apple from throttling your iPhone

A Reddit post from December got the

a

ball rolling. It highlighted an issue where processor performance on iPhones was decreasing because of battery age. Battery capacity is generally expected to decrease with age but the iPhone’s performance should stay the same. A blog post on GeekBench explored the issue further, confirming that once a battery was replaced on an iPhone 6s or 7 running an up-to-date version of iOS, the iPhones performance noticeably improved. An iFixit engineer also found similar results when they performed tests.

Apple confirmed on December 20 that it was throttling iPhone performance as iPhone batteries age and their capacity deteriorates to avoid other performance problems. It followed this by publishing a public apology letter to customers on December 28.

"We know that some of you feel Apple has let you down. We apologise," the letter says. "There’s been a lot of misunderstanding about this issue, so we would like to clarify and let you know about some changes we’re making."

http://www.wired.co.uk/article/iphone-slow-down-battery-replacement