And the iPad 3 and 4 perform even worse than the Air when running iOS 8 and 9. System requirements don't stay the same forever. Like it or not, iOS has become more and more bloated as generations pass. The A5X may have been fine running iOS 5, A6X with iOS 6 and A7 with iOS 7 but there's no guarantee it'll run updated firmware well. Indeed the alternative is far more likely to occur.Also I know about the resolution difference in the iPhones versus iPads but did the A7 really wasn't powerful enough to drive that 2048x1536 resolution ? I highly doubt that. The only chip that was not powerful enough to drive the Retina display was the A5X. A6X and above were perfectly capable of pushing all those pixels so the first gen iPad Air should not perform like crap as it does now on iOS 8/9 (assuming it was indeed much better on iOS 7).
I've got an iPad 4 running iOS 6 and in terms of responsiveness/UI, it actually compares favorably to the iPad Pro 9.7 running iOS 9. Too bad lots of apps have stopped supporting iOS 6 and there are quite a number of features I use (control center, call and SMS forwarding) missing from the older iOS version.