I am currently on ios 12.1.1. It is not too slow (chrome opens in 5 seconds). But Ios 11 was even slower. Is any ios after 12.1.1 faster on mini 2? Maybe 12.4 is faster?
If you update you'll get 12.4.6 which is the latest version for your device. I doubt you'll see any difference in performance only bug fixes and security improvements