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I only get 8 hours on my 6s plus, that's with everything turned off location, 3D Touch, Siri, Apple Music, LTE, iCloud, email... Better than my 6s which only got 5 hours.
 
Yeah the battery life on the iPhone 6s Plus is not that great either. I get around 8-10 hours of usage time. I thought it was rated better than an iPad but my Air 2 gets around 16-20 hours of usage.
 
Yup I tried the latest version. I even tried the latest public beta (9.3) and same results

The "usage" statistic the iOS settings tells you is more than just when your screen is on. You said you turned off a whole bunch of stuff but iOS counts all that stuff you turned off as "usage". When you turned all that off, most of your actual usage is with the screen on (the screen when on probably takes up the most power).

The usage statistic is very misleading. It isn't counting the minutes you use the phone, it counts the minutes the OS is active. iOS counts the minutes of the phone playing music with the screen off as equal to the minutes of when the screen is on max brightness using the cpu with max load.
 
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