From Joanna Stern’s iPhone 6s review for the WSJ:

Let’s get this out of the way first. The No. 1 thing people want in a smartphone is better battery life. And the iPhone 6s doesn’t deliver that.

And:

Still, there is no battery improvement over last year’s iPhone 6 models. In our grueling test; which loops a series of websites with brightness set at around 65%, I found the 6s and 6s Plus get no more—but also no less—battery life than their predecessors. (iOS 9 itself adds an hour of savings to the iPhone 6, and has Low Power Mode.) Strangely, although Apple says that the 6s Plus gets hours more battery life than the 6s, repeated testing on multiple devices reveals a slimmer difference. In a Web surfing test, the 6s on average lasted for 8 hours, while the 6s Plus went 20 minutes longer. In a video playback test, the difference was an hour.

This is a common theme in reviews I’ve read this morning. Given that Apple had to ship a smaller battery to make room for new components, the fact that the 6s is still getting the same performance on a more powerful hardware suggests that iOS 9’s optimization technologies are working.