It's got nothing to do with some people noticing issues and others not. The black and white is that some people have issues and others simply don't. I don't have any issues with my iPhone 6 although I had some before a reboot. It's not that I can't see issues, it's just that I appear to be lucky and don't have them.These people write off our experiences with a scoff and a hint of condescension, yet we've to believe their opinion? Absolute hypocrisy, telling us that iOS 9 is absolutely fluid and smooth and that we're wrong. There is no right or wrong here, that's what these people don't grasp. Some people have a level where the animations are acceptable. It's not medical or neurological like some other posts I've read, it's simply that some people can't see the drops and stutters and some (many judging by this forum and others) can.
When you've been accustomed to 6 months daily use of iOS 8 on iPad Air 2, it's very easy to notice any drops or stutters in the UI animations. That device is, until the A9 and A9X devices launch, remains THE optimal iOS experience.
The way you can pinch to home, immediately tap an app icon, pinch to home again immediately upon the opening, tap a different app, etc is ridiculously responsive and quick. On iOS 9 I simply couldn't do that quickly, and for some reason the pinch to home gesture kept resulting in the swipe left or right through open apps gesture instead.
No stutters, drawn out animations masking the lag and no freezes. Others may have them but I don't. I hope Apple release an update soon to correct it for everybody.