iOS 9's battery-saving mode will slow down your iPhone
Benchmark tests showed that without Low Power activated, the iPhone 6 Plus scored 1606 on the single-core processor test and 2891 on the multi-core processor test. When Low Power was then turned on, the same handset’s scores dropped to 1019 and 1751 respectively.
Results were similar when the test was conducted with an iPhone 5s. Its performance was reduced by around 40% in a bid to conserve battery.