For infinite speed, VSB gave the rating to certain characters if they were able to move in a void where time did not flow (or exist in it) or something like that. It's really weird because wouldn't that kind of feat be something similar to time stop resistance or just plain being unaffected by abilities that manipulate with time in some way?
Anyways...
If equation for velocity is distance over time, then infinite speed should be something like a character managing to cross an infinite distance (if a universe is said to be infinite in size, then characters going in an out of it and towards other existing dimensions should qualify).
For immeasurable?
It'd need to be a kind of speed feat where the standard velocity equation cannot apply, such as the distance not being quantifiable (in this case, characters who are larger than two or more infinite-sized universes including the distance between each other)...or time not being quantifiable, such as being able to interact with different points in time without any type of time manipulation due to said character's existence being outside the flow of it.
@Edward Nygma You got characters who can move in an out of universes & feats involving entire multiverses which pretty much ignore what is explainable with conventional math/science.
So in a way, with characters being able to perform feats that break outside of the boundaries (size, flow of time, & any other unknown variables not proven) of our own universe, rules for math & physics which are confined to our own existing universe stop applying at that point.
True form of Darkseid falling being able to collapse the entirety of the DC Multiverse? How would you even quantify the actual size of that thing (the DC multiverse has an infinite number of universes, doesn't it?)
But still, it's a feat that happened so...