Looking at Qualcomm's chip comparison site, it's interesting HTC chose this particular chip when they could've chosen others, but I think they were really looking for something they could optimize around performance and battery life. The 615 can actually handle higher resolution displays but the 617 made more sense because they were only outfitting the phone with a 1080p.
Snapdragon 617 running 8 cores with 3gb ram. How the he'll is a 2150mAh battery going to last.
Or is this 617 really battery efficient? They didn't have to put in a bigger battery.
The 617 is supposed to be battery efficient, as is the AMOLED screen and Marshmallow. They couldn't put a bigger battery in because of the thinness of the phone. I have an M7 and it's thin to me. If they had made the A9 as thick as the HTC One M7 they could have put a much bigger battery in it and had a rockin' phone that was still thin when compared to the competition.