In short — it’s bad.

In iOS 9 on the iPhone 5 and the iPad 3, I wrote:

The range of devices supported by iOS 9 is surprisingly ample, the principle being that if a device was able to run iOS 8, then it’ll run iOS 9. Performance-wise, according to what I’ve been reading on the Web so far, the consensus seems to be: If you were satisfied by iOS 8’s performance on your (older) device, then iOS 9 won’t disappoint. I agree with this assessment, and I’ll go as far as saying that iOS 9 feels even smoother and more stable in places where iOS 8 stuttered every now and then. I noticed this on the iPhone 5 especially.

That, sadly, isn’t true for the iPad 2.

At the time, I left the iPad 2 out of my initial impressions because I didn’t want to be too precipitous in my assessment. I don’t use an iPad 2 on a daily basis — my wife does, so I just told her: “Let me know if you encounter any issue with iOS 9, and how the overall performance is.”

Time passed and I didn’t hear many complaints from her, so I assumed things were generally fine. However, it appears that every iOS 9 minor update released after version 9.0 has negatively impacted performance and usability on the iPad 2. I took advantage of the Christmas holidays to take a closer look, spending more time with her iPad 2. Now I can definitely say that allowing the iPad 2 (and by extension the iPhone 4S) to run iOS 9 was a grave mistake on Apple’s part. 

• Animations and transitions are a sad spectacle to behold: stuttering, sometimes even stopping midway for a couple of seconds; it really looks like the device is struggling to complete them. Every now and then, the lock screen is unrensponsive, meaning that you can slide to unlock all you want, nothing happens. Invoking Control Centre and Notification Centre can be a hit-or-miss affair. On a bad day, even swiping from one app screen to another is comically slow. The fluidity of the multitasking interface is barely acceptable.
• Keyboard input is frequently lagging. You type in a search field, or in Safari’s address field, or you just want to type a quick note: you start tapping the keys, nothing seems to happen, then most of the text you’ve entered is finally rolling down all at once, machine-gun style.
• 512 MB of RAM are simply not enough to handle iOS 9. This is noticeable everywhere: Safari tabs constantly reloading; applications losing their state after you quit them and reopen them after a short while; and finally, I know the practice of entering the multitasking interface and force-quitting apps is frowned upon, but on the iPad 2 it really looks like it helps give the device a bit of breathing room to operate. Maybe it’s just a subjective impression, so take it with a grain of salt.
• I’ve noticed that apps have taken to unexpectedly quit with an alarming frequency. Even apps that shouldn’t have any compatibility issues with iOS 9, either because they’re first-party apps, or because they’re freshly-updated apps. I’ve also noticed that in a couple of occasions the iPad 2 self-rebooted out of the blue. I haven’t been able to reproduce the issue. It all just seems so random.

It’s really a pity that Apple makes it hard to downgrade to a previous version of iOS. I know, security reasons and all that, but downgrading to iOS 8 would really help regain some fluidity and overall performance on the iPad 2 (and the iPhone 4S, I guess.)

Speaking of iPhone 4S, Rich Edmonds at iMore reports that Apple faces lawsuit over how iOS 9 negatively impacts iPhone 4s performance:

More than 100 iPhone 4s owners have asserted that the update has led to a negative experience on their smartphone to the point where it’s simply unusable for daily use. Delayed launch times for apps, slower response on the touchscreen, as well as sluggish performance overall plague said owners. Then you have the reported freezing and crashes. Not fun times for anyone.

From my recent direct experience with my wife’s device, I’d say this is also exactly true for the iPad 2.