I still use an iPad Mini 2 on a daily basis (including today to take some notes) and it's on the Public Beta of iOS 12 currently.
Before that It's been through everything since 7.0 when I got it. I kept it on 9.3.5 for an extended time and jumped to 11 and it was just as bad if not worse than 9.x. However, iOS 12 is much faster than 11.4, and in fact I'd say it's at least as quick as 9.3.5 if not slightly better. It won't Match iOS 7 or 8 of course, since there's a whole lot more running now and newer features since then.
But IMO iOS 12 does run at least as good as iOS 9.3.5 if not better. It's primarily the 1GB or RAM that holds us back, more so than just the A7 chip, as it's not that different from the A8 CPU wise, and the mini 4 is much, much faster with it's 2GB of RAM. Same with the Air 2 and it's A8X.
Plus the gestures inherited from the iPhone X are just plain fun.