Low Power Mode allows iPhone users running iOS 9 to eke out more battery life when the battery is getting low. It doesn't just pull this ability out of thin air, it causes iOS to turn off certain features like mail fetch, background app refresh, automatic downloads, etc.
What's more, Low Power Mode significantly reduces the clock speed of your iOS device. We've seen before and after benchmarks with older iPhones involving Low Power Mode, but what about Apple's just-released iPhone 6s? How much of an impact does Low Power Mode have on it?