I finally used an iPad Air 2 today and it was amazing. You can actually slide down for spotlight, start Siri, use control center on the lock screen, use control center with an open folder, and it doesn't stutter. Every one of those situations are a mess on my one year old iPad mini 2, exactly the same on my friend's air 1. Super disappointing and frustrating.
Same as when using Safari in landscape mode and you tap the address bar (whilst currently on website, not a new tab), and the current address slides left while the keyboard slides up from the bottom. On iPad Air and iPad mini 2 this has some stutter. It's not unusable but it's noticeable. The only way to remove it is to disable transparencies which means the home screen dock looks ugly as hell. What causes this is the stupid transparent box that pops up with favourites or bookmarks (depending on your settings) which mirrors the Safari UI on Yosemite. On a new tab in iOS 8 these show as they did when you tapped the address bar on iOS 7, a blank white page with the icons of your bookmarks. Why add the useless transparent box with the icons?
Let's be honest though, this is just Apple gimping A7 devices. App performance, graphically, on both devices is identical in every top end game. Look at titles like AG Drive, Asphalt 8, etc which run identical on both the Air 1 and 2 using Metal on iOS 8 to run better looking games than ever on iOS (AG Drive needs to be seen to be believed).
If the Air can run these kind of games, then there's no reason for some stutter when a layer of blur is added to a home screen or keyboard in Safari. Can't Apple apply Metal to their own bloody OS on iOS 8 enabled device to eliminate these moments of stutter.
I run my Air in Reduce Transparenct mode because the Safari stutter just annoys me. I tried out an Air 2 in Tesco and it was as smooth on the above Safari situation as my Air 1 is with reduce transparency enabled.
However I refuse to believe the A7 chip and 1GB of RAM is incapable of handling a layer of blur when the thing can throw polygons around at 60fps in someo of the most amazing looking mobile games to date.
The thing is that every other animation is identical to the Air 2. The pinch to home screen is fluid and smooth, activating multitasking is quick and responsive, swiping apps away is smooth, swiping between open apps with gestures is smooth also. It's just places where this blur is applied, yet in iOS 7 where there was blur the iPad Air was fine. Spotlight used to show the home screen with no blur behind the keyboard and search bar on iOS 7, why change it? It's just adding resource hogging graphical nonsense for the sake of it.