DLed iOS9 this morning on my Air Mk1; anecdotally, things don't seem any slower, and in fact, the GMail app seems to run more smoothly now than previously so far.
Biggest thing for me is the Safari ad-blocking (i.e. it may actually get me to start using Safari now) although I think the way the ad-blocking extensions are handled is extremely clumsy to the point of being dumb. There is nothing built-in which tells you that you first need a third-party ad-blocking app, and then that you need to activate that app from inside Safari to block the ads - this is utterly, utterly un-intuitive.
I've tried the dedicated Adblock Browser on Android and iOS, and it's simply not a good browser - Chrome, with ads, is actually faster and smoother in use than the AdBlock Browser is whilst natively blocking ads. Who knows; Maybe Google will step up with a modified version of Chrome in response.
