Apple decreases CPU speeds to preserve your old battery.
A code introduced in iOS 10.2, called ‘powered’, apparently reduces the processor speed of your iPhone depending on the battery age in order to prevent unexpected shutdowns. The creator of Geekbench has found a strange correlation between the battery age and the CPU performance. In addition to a weaker battery life as the device ages, which is normal, different iPhone models register lower-than-usual scores at Geekbench 4. Interestingly, if the user replaces the battery, the score increases and the CPU performance is back to normal.
powerd definitely has the power (haha) to reduce processor speed pic.twitter.com/YK7xqy8KGW
— Guilherme Rambo (@_inside) December 18, 2017
The distribution of scores for iPhone 7 depending on the battery age is the same between 10.2 and 11.1.2, but with iOS 11.2.0 the performance looks similar to the iPhone 6S running iOS 10.2.
There is a strange correlation between the device /battery/ age and the score, and this appeared after a change in iOS. According to the publication, Apple has changed the CPU-limit performance when the battery degrades to a certain level, introducing a hidden third power mode – a state between full-power and low-power. Whether an iPhone is performing poorly because of a reduced performance or not it’s hard to find out, because Apple is not informing users with a notification that their iPhone is in this third power mode.
Find the full article with some graphs here.