For infinite speed, travelling an infinite distance in a finite amount of time via speed alone (Bow of Auri-El & the movement of the plane(t)s in TES, Simon escaping the Labyrinth in TTGL). For immeasurable speed, you have to do the same but with moving through time as well as space (the Shrike from Hyperion Cantos). I think irrelevant speed is when speed as a concept is irrelevant, and you have to be some kind of god, which is much harder to quantify.