iOS 9.3 is doing great so far. Well, beta 1 was great, beta 2, not so much. I'm assuming it's just a blip since it's still in beta, and it will be fixed. Beta 1 was a whole worlds difference on my mini 2, for the first time in my iPad's lifetime (I have had it since iOS 7.0.3 I believe is what it was on) spotlight was perfectly smooth. Control center improved dramatically, keyboard was a bit smoother/snappier, opening apps was quicker, app switcher was almost perfect. You could sit at the home screen, go to the app switcher, and go back and it would all be a perfect 60fps for the first time. However if you were going into the app switcher from an app rather than the homescreen, it still had it's usual hiccups. Better than 9.2 still though. This was with beta 1.Now with iOS9, unlike other iOS releases, for the first time, Apple took the boldness of claiming "Under-the-hood refinements bring you more responsive performance..." and it actually ended up being worse. This is why I believe in part, all the user outrage is rightfully justified, and Apple deserves all the heat they got on it.
I also believe Apple deserves all the pressure/criticism because not only their advertising was misleading/wrong but they also forbid users from downgrading to a previous iOS.
So if I upgraded to the new OS believing that Apple's advertising was right, I'd be screwed because I can't downgrade. I mean sure, you can downgrade for like 1-2 weeks after release, but still, that's assuming one upgrades the same day the OS is released, and that's assuming you realize there is something wrong within those 1-2 weeks for which you can't adapt/compromise, otherwise you are screwed.
Beta 2 though, iPad mini 2 is once again a wreck. Horrible notification center now, bad app switcher again, bad spotlight again... iPhone 5 went downhill too, now it has a laggy control center, spotlight, and notification center, where it all was previously beating the iPhone 6 when both were on 9.2. Really hoping they just readjusted Metal and OpenGL, and they clean it up next time around or at least but public release. They made a huge step forward but then took two giant steps back. Really disappointing. iPhone 6 on the other hand improved with beta 2, I've heard similar reports about the 5S. Now control center is a perfect 60FPS just like in iOS 8, even over messages, open folders, app switcher, and the lock screen to name a few previous problem areas. The popup animation for airdrop is perfect everytime, which even the 6S on iOS 9.2.1 can't achieve. iPhone 6 Plus took a hit though however, similar to my iPad apparently.
Even though we took a hit on beta 2, 9.3 is bound to be a great release in terms of performance. Possibly finally bringing the promises Apple originally made. Just judging on how beta 1 was so good, we are bound to finish out in an even better or similar position in terms of fluidity. Beta 2 is just a blip in my opinion. It'll be fixed. And iOS 9 will be smooth finally.