Safari, on iOS 8, was a daily crash machine on my 6+. Very frustrating and not reliable for my day to day usage, since Safari is my preferred browser. I haven't experienced one Safari crash on iOS 9. Safari is now a joy to use.
That alone, regardless of any lag or perceived lag, is worth upgrading in my opinion. I remember what a mess iOS 8 was when it was released, so to me iOS 9 is on a much better foundation up to this point.
A couple of more updates and who knows, maybe the 8.4.One crew will move to iOS 9. Tried to make a funny, but not so good at it as well.