Based on anecdotal observation, many iPhone users have long believed that older iPhones get slower over time. Generally, people have assumed that this is because of new features and additions in new versions of iOS that are better optimized for the latest phones.
But Reddit users, and Geekbench developer John Poole, have a compelling new theory, backed up by benchmarks: the iPhone may throttle performance to preserve battery life or avoid unexpected shutdowns as the battery degrades.
Several days ago, Reddit user TeckFire posted a report to the iPhone subreddit stating that, after experiencing slowdown on their iPhone 6S, they replaced the battery with a new one and saw significant improvements in benchmarks—seen below, via their imgur post:
Source:: Arstechnica – Gadgets