Well, this is something that really bugs me. I had a 6s and then returned it and got a 6s Plus. And I noticed that the animations looked... choppier. I compared next to my fiancée's 6s and sure enough it looks like all my animations were running at 30fps while her's were running at 60.
I found this
http://www.cultofmac.com/394468/how-...-rate-problem/
Which has a "fix" for it and a video.
(the video shows whats going on (you have to make sure you view it at 60fps to see the difference)
Basically, Apple has coded everything to work best on the 6s instead of having it work best on their most expensive device (the Plus) and then scale down. Since the Plus is running a higher resolution than the 6s, it can't keep up. This is pretty ridiculous to me. Why not code and design it for the Plus Apple? Then it will guaranteed work perfectly on the regular model.
The "fix" the video shows is if you toggle the "reduce transparency" option, the animations play at 60fps. And sure enough they do. But this is a really frustrating choice. Do I want my OS to look cooler? Or operate smoother? The fact that I have to make the choice when I have the most expensive phone they offer (128gb 6s Plus) is pretty annoying.
/rant