I never played with a 6S, but my 6+ has been absolutely great and fast with iOS9 and now b4 9.1 even better. My iPad Air 2 is doing much better as well. Both use much less memory and evrything is faster and never crashes.
I don't enable lots of junk on Settings and I take good care of what is installed. There's only 1 gb RAM, so there are no miracles with that. I think the planned obsolescence is due to adding new features to older hardware, on top of what already was available. If the iPhone 4s had 2 gb ram, it would still be a fast and powerful device. Except it has only 512mb.
I just installed 9.1 Beta 4 and my 6+ is flying also. Very cool.