Well, numbers don't matter in the end because most of this is just a matter of perception. You can show me that Booting is much slower on iOS9 than iOS8, but I so rarely boot my phone that I don't care. Apps start much faster. The keyboard pops out slightly slower. There are differences and if you want to only measure the things that are slower, you'll have cold hard numbers and still a flawed comparison. So measuring things is only part of the experience.
If iOS8 was almost unusable on your iPhone 4S, then I'm sorry. I've lived with it for about a year and found it fine. It was sure not as snappy as iOS7, but it brought some cool things and I found the deal acceptable.
As far as battery life is concerned, I can go full-day without recharging now which was not the case with iOS8. True, some of it is due to the fact that whan I reach 20% of battery, the iPhone switch to "energy saving" mode (or something like that) which did not exist in iOS8. It may be artificial, but it works. And it works well.
So, as I stated, and this is a subjective opinion, if I could switch back to iOS8 ot iOS7, I wouldn't. I'd keep iOS9. The rest is irrelevant.