If you are coming from an iPad 4 (which by the way seems more and more laggy everyday), would you notice a big difference if upgrading to the Air 2 vs Air? Is upgrading all the way Air 2 worth the extra $100?
I would do a backup and restore to factory first (the analogy of "reformatting" your iPad) to see if it improves things.
I think the A6 still has some juice left in it for iOS9. What I'm worried about is iOS 10 forward, where Apple might focus on 64bit SoCs. But then you might be able to wait for the Air 3.