Yeah my mini 2 is sort of pitiful with iOS 8 and laggy UI, mostly because of the translucency effects. One tip I have for you is to keep zoom turned off. For some reason it was plaguing my iPad with lag, rotation would take forever and you would see a black border rotating, THEN the actual content rotating. On the lock screen it does the same thing except twice as bad, with "Slide to Unlock" lagging at 1-5 fps. Exiting safari or messages (especially with the keyboard open) would cause a 20FPS closing animation, it was really sad. Turning zoom off believe it or not fixed these issues. However, even with zoom off there is still a ton of lag regarding any situations with translucency (Aside from Notification Center and Control Center on the 'Home Screen') exiting apps, entering apps, going into or out of app switcher-there is occasional moderate lag. All if it is incredibly sad. A lot of this even existed on 7.1.2.
Anyway, as the iPad mini 2/Air are obviously underpowered, I still solely believe that a ton of iPad optimization is still definitely needed. The amount of bugs that exist on iPad (aside from lag) is astronomical when compared with an iPhone.
Oh- doesn't the A7 also have quad core graphics, essentially making the A7 equivalent the the A6X in that regard? The A8X has like 6 or 8 cores for graphics I think, but I'm not totally sure on that.
Also, I'm curious-how does the Air 2 handle the translucency effects? Does the keyboard and blurring of the springboard lag when dragging down for spotlight? Does the Siri animation lag? Does rotating the iPad with the keyboard open lag? Is there HORRENDOUS LAG when dragging down the Notification Center or Control Center over the keyboard and translucent "Favorites" area in Safari (When typing in the search field)?) Is there lag when activating Siri with the keyboard open? Is there lag when activating the Notification Center and Control Center over the keyboard?
None of those areas lag on my mothers iPhone 5, or my iPhone 6, but every single one of those instances causes heavy lag on my Mini 2 and my friends Air. They actually are smooth on my other friends iPad 3 because the translucency effects are disabled; It is only a slightly transparent black or gray overlay.