Okay, let me say this, I have a pretty big family, myself, my wife, brother, sister and my mother all own different versions of iPhone 6. Myself, my wife and my sister own iPhone 6 plus, while the rest are regular sized iPhone 6.
I upgraded all of them to iOS 9, as soon as it came out and all our iPhones have become extremely laggy.
There's frame-drops when I pull down spotlight search, when scrolling through apps. Also launching apps takes noticeably longer. Screen rotation is also extremely slow.
This is present on all iPhones that my family owns.
Now, having said that, my mother and my wife doesn't notice any difference in performance. To them it seems like it's just as fluid as iOS 8.4.1.
It seems like some people can't tell the difference between 60fps or 12fps and etc. It's just like there's variation in people's vision, there's also variation in people being able to perceive frame rates.
I have always thought of Apple as a company who takes great pride in paying attention to small details. In this case, iOS 9, makes all our iPhones run like Android. I just can't believe someone at Apple said "That's good enough, let's release this laggy OS to public".