Personally, I feel like my iPad Air is less laggy and glitchy on iOS 9 vs iOS 8.
The iPad Air on iOS 8 was just so frustrating with continuous "A problem occurred with this webpage so it was reloaded" errors there were times I was tempted to throw it at the wall. I almost couldn't wait for the Pro 9.7 and came very close to buying an Air 2 last year. iOS 9 feels like it gave my Air a new lease on life. I still get some page crashes on iOS 9 but they're much rarer now and at least Safari doesn't go on a continuous refresh loop.
Of course, ymmv.