Problem is the SD810 and the 3500mAh are to compensate for it.. Sadly this year the 810 is killing loads of phones battery.
If you still don't believe it, take a look at Xperia phones battery benchmark.
Read more at http://www.phonearena.com/news/Goog...tive-use_id74853/comments#p4Wzie2yTPrFWxtS.99
That's completely false! Nexus 6P is not using the same SD810 as in Xperia you mentioned.
The Nexus 6P uses the latest version 2.1, which comes with improved power consumption and fixes overheating issues.