Oh I have no doubt that iOS 9, by the end of its lifespan will be superb in terms of UI fluidity. I remember how bad iOS 8.0 was on my 5s, which was superb on iOS 7.1.4.
It took iOS 8 time to mature into the OS it finally was, but my issue is that Apple release a .0 build with these issues to begin with.
It's a common theme and has been for the past few years that iOS is released pretty buggy/laggy/stuttery on devices other than the newest iPhone, and it takes Apple the best part of a year to make the OS it's very best on recent devices only to replace it with a new version, and the cycle begins again.
My only issue with iOS 9.1 (and 9.0.2) is the scroll stutter, which occurs randomly and in certain apps. iOS 9 should be smoother and more efficient due to Metal and its smaller footprint, so I'm optimistic about future updates. But that doesn't stop current issues being annoying when your old OS had none of them.