It looks as if Apple made good on its promise to make iOS 12 run faster on older iPhones and iPads. Max Yuryev did a comparison:
This first iOS 12 beta breathed new life into our iPad mini 2, which was almost unbearable to use under OS 11. If you’re someone with an older iOS device the ultimate release of iOS 12 is great news, and we applaud Apple for their focus on performance and not dropping compatibility for older devices. The results were a bit mixed with our iPhone 6 but very impressive with our iPad mini 2.
And that’s on the first beta release.