The first version of iOS11 sucked big time on the Mini 4. The interface animations were a miserable lag fest. Later versions of iOS11 improved the fluidity a lot.
I took the plunge on iOS12 and I'm very impressed. It runs extremely smooth considering the fact that the Mini 4 was released in 2015. Battery life might have worsened slightly, but it's just a feeling. My devices has clocked +300 cycles, so it's not really new anymore.
I'm very pleased that Apple for once has optimized the performance on iPads. The Mini 4 doesn't really feel like a device released back in 2015.