I found this video interesting in that it shows a 4 year old iPad 2 performing quite well on iOS 9.1. Doesn't keep up with an Air 2, obviously, but still quite usable.
Depends what you find usable I guess. I'm ok with mine, but iOS 9 was shocking for it. It struggles keeping one safari tab open, let alone two. My Dad's iPad 2 on iOS 7 is much better. iOS 8 was somewhere between the two. iOS 9 seems to have less than half the available (Free) RAM that iOS 7 had when idle.
I think also the number of apps you have plus other things like the number of mail accounts, what your iCloud settings are etc have a massive impact. I had hope that iOS 9 would return to iOS 8.4.1 speeds after a few updates, but sadly it hasn't, and honestly, besides security I would have a hard time justifying suggesting to someone else to upgrade from 7 or 8 to 9.
I hope to get another year out of mine given the disappointment that the Pro 9.7 is. If you don't expect it to cope with multitasking and you don't do much web browsing the iPad 2 is ok.
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"cripples" them, but I have to say that this doesn't seem consistent with what I've seen in my (admittedly limited) use of older iPads.
The iPad 2 has also been one of the best ageing iPads. Mine was fine with iOS 5,6,7 and only started getting slow with 8 - 9 has been the real killer. I think my annoyance with 9 is that it adds so little especially to these devies that should make it slower, yet it is, with apps taking longer to load every time, animations jittering even more and more reloading of tabs and apps in multitasking. Other iPads (like the Mini 2 and iPad 3) have aged worse.