Can the people who have phones who don't exhibit issues also not jump in every time someone DOES have an issue and claim that since they don't, nobody does? The truth is somewhere in the middle. I have a 6+ 64GB that exhibits stuttering while scrolling, choppiness within the multitask switcher, the list goes on. I have restored and set my phone up as new, something that is a complete and utter pain in the ass, all to "go through the steps" of what everyone who doesn't have these issues claims will fix everything like magic.
It's also well documented that the iPhone 6+ seems to get the worst of the performance issues here, as for what exact reason nobody seems to know but my thoughts are the hardware that was touted as being the best in class only a year ago can't handle what iOS 9 is doing. The real problem is however that how can any of us come up with a solution to this problem? We can't. This is on Apple to make things right, not us. If the only proposed solutions to me are "deal with it", "just go buy an Android or Blackberry idiot", or "well my new S model doesn't, so maybe get one of those", then I guess I have to just "deal with it". There's absolutely zero reason to think that hardware that's only a year old that ran the last version of iOS at blistering speeds is now so bad that it can't handle the new iOS.
Finally, we all get that nothing has been or ever will be 100% perfect, but simply that when Apple goes on stage, at their main OS event, to espouse that they've optimized iOS 9 to be faster and smoother than ever, when it's in reality NOT that on some devices (my 6+, which is laughable), is bogus.
The end.