I have an iPad Mini 2 running iOS 9.3.5 which I would like to upgrade to iOS 12 so I'm sifting through everyone's posts to determine if I should stay on 9.3.5 or upgrade to 12.
I would be happy to post my experience going from 9 to 12 (which might provide everyone with a clearer picture on performance changes/differences) but I'm being cautious .... perhaps too cautious?
iOS 9.3.5 has always been a bit laggy on my Mini 2. I guess if iOS12 responsiveness is the same as 9.3.5 then that would be quite acceptable. I would be happy with that.
Should I take the plunge?
Okay I took the plunge.

Responsiveness is actually pretty good. Where it lagged for me in 9.3.5 it no longer lags.
However there are a couple of new places where the lag is now noticeable.
From the lock screen when you swipe to access the camera, that is really laggy. Swiping up to bring up the Passcode screen is a little slow.
From the Home screen when you swipe up to view running apps, that doesn't work too well. You have to swipe up rather slowly in order for the apps to appear. And you can't swipe down (I guess you would call it "peek view", as you can in iOS 11) to go back to the Home screen.
Edit: I figured out that this was due to "Reduce Motion" being on. Since switching it off it's much better. It's slightly different from iOS 11 but only because iOS 12 has changed the way this functions. So it works as expected, as it should.
Swiping through app screens, swiping down notifications etc are all quite responsive.
Force touch (??) functionality on the options in Control Center is a little slow.
I have iOS 11 running on my iPad Mini 4 and it works perfectly. I much prefer iOS 11 to iOS 12, especially where you swipe up to reveal running apps. It just works much better and smoother.
I checked all my photos, notes, apps, books, personal files in various apps and all were restored after the update without any issue.
So considering it wasn't a clean install, performance is quite acceptable.
So if anyone is still on iOS 9 or above on their iPad Mini 2 then I would recommend updating.
The new features are worth it.