I have no clue how you measure the "success" if an operating system?Not really true - there were lots of irate threads on here ast year complaining about lag on the iPhone 6/6+ in iOS 8 (e.g. the weather app and some screen rotations).
As has been pointed out, performance is fine running high-end games, but not when doing mundane OS things. It is simply piss-poor optimisation and QC by Apple. I am glad it is this way around (I would rather a few meaningless OS transitions be jerky than app performance be bad), but when you've paid hundreds of $/£s for the latest hardware running an OS that's been touted as being optimised for performance it's not great and something that Apple used to trade on. Whilst iOS 9 hasn't had the bad mainstream press that iOS 8 got, I still don't think it's been a success for them.