iOS 10 and 11 were definitely guilty of making older devices slower, but Apple effectively admitted that was a mistake since one of iOS 12's big selling points

iOS 10 and 11 were definitely guilty of making older devices slower, but Apple effectively admitted that was a mistake since one of iOS 12's big selling points is that it improves performance for older devices. iOS 12 supports all the same devices as iOS 11, nothing was left behind.