It's a fact of life that the more you ask of a device, the slower it will become. And since every new release of iOS brings with it new features, it's only logical that these new features are going to put additional stress on the old hardware.
See also: Is the iPhone just too darn expensive?
Because of this, some owners of older devices don't upgrade their iPhones or iPads out of fear that their devices will suffer performance issues. Right now some 19 percent of iOS devices are running iOS 8, despite the fact that every device that runs iOS 8 - iPhone, iPad, and iPod touch - can be upgraded to iOS 9.
And one major downside of holding back on upgrades is that these iPhones and iPads aren't receiving much-needed security updates. Even if you decide to stick with 9.2.1 afterwards, that jump will have plugged up a lot of security holes in your device.
Well, iOS 9.2.1 might be the upgrade that iOS 8 users have been waiting for. According to a video published by iAppleBytes, these older devices might actually benefit from being upgraded to the new iOS release.
While boot times will be slower - unless you're running an iPhone 5s, the first iPhone to feature a 64-bit processor - overall the older devices are snappier on iOS 9.2.1 than they are on the last release of iOS 8, which was iOS 8.4.1.
Although I've not done side-by-side comparison tests, I can confirm that I too am seeing an increase in performance on older devices. It's not a huge "blow your hair back" increase, but it has made older devices feel snappier and a bit more responsive.