"With the Apple-designed A9 chip in your iPhone 6s or
iPhone 6s Plus, you are getting the most advanced smartphone chip in the
world. Every chip we ship meets Apple's highest standards for providing
incredible performance and deliver great battery life, regardless of
iPhone 6s capacity, color, or model.
Certain manufactured lab tests which run the processors with a
continuous heavy workload until the battery depletes are not
representative of real-world usage, since they spend an unrealistic
amount of time at the highest CPU performance state. It's a misleading
way to measure real-world battery life. Our testing and customer data
show the actual battery life of the iPhone 6s and iPhone 6s Plus, even
taking into account variable component differences, vary within just
2-3% of each other."