i feel like it opposite ... i suppose once the animation starts it looks good and smooth, but it appears to take a while for the animation to start
e.g. opening an app, switching to another app, doing just about anything in imessage
I'm on an iphone6 though ... so maybe the hardware is "outdated"
I'm on a 5S, which is even slower than your iPhone 6, but it feels faster to me in some spots when compared to iOS 9. But I already felt lags in iOS 9 so maybe I'm just used to those. Mind you, your iPhone 6 is bigger than the 5S, and some people have noticed that the 6 Plus actually felt slower than the 5S at launch. Not so much with the 6, but nonetheless I think screen size has something to do with it since it takes a bit more compute power and more memory.
I don't know how applicable this gfx bench is, but it does illustrate what I'm talking about:
In this very specific bench, your iPhone 6 basically performs similarly to my 5S, and actually is slightly behind it, whereas the 6 Plus loses badly.
In contrast, I've read a review that says the iPhone 7 Plus is just about perfect in terms of OS feel (if you exclude the bugs). What they meant by that is that with every iPhone release so far in recent years there have been occasional stuttering or lagginess. Most people didn't really notice it, but he did, even with the 6s series. He doesn't with the iPhone 7 series. It's just perfectly fluid.
Overall though, we're just nitpicking.