I personally felt iOS 9 is better on iPhone 4s than iOS 8 was.
Not everyone shares the same opinion, but I do feel the animations are much smoother, the multi task app switcher is a bit quicker, and there is not as much animation lag as there was on iOS 8.
In speed tests, iOS 8 may be better and launch apps faster, but it felt like it jittered and stuttered. iOS 9 resolves those animation issues and makes it feel better overall
Its not as good as iOS 7 or especially iOS 6 was, but its certainly a usable device for your basic needs.
I personally felt iOS 5 is better on iPhone 4S than iOS 9 was.
Because really, are all the new features in iOS 9 more valuable than having a responsive and smooth user experience?