Idk how to quantify the speed exactly, so I'll just give you guys some ideas.
First Interpretation: The sword emanates down from the higher-dimension, effectively "teleporting" into the universe and then cleaving across it.
This is personally what I believe. The speed would depend on where it begins in the universe (where it first teleports to).
At the low end, this would be the edge of the planet's atmosphere (or somewhere arbitrary in the sky above the clouds).
That is where we first see Saitama notice it. (Though you could assume that to be WAYYY further out if you wanted to) The speed would be pretty abysmal with that being the case.
At the mid end, you could assume this to be the edge of the universe itself. We see on multiple panels that
there are galaxies in the bubbles realities. If the sword first enters at the edge of the universe and then begins to travel towards the location, then this would be ridiculously high into
MFTL+ territory, if not just outright
Infinite (At least for the initial strike).
With the highest end interpretation, you could say that the sword is literally just traversing the space-time itself in order to "teleport." Would get you to
Infinite or
Immeasurable speed. Though it seems to be a bit contradicted. You could say that the initial strike down is infinite/immeasurable (or just teleportation) but then him cleaving across takes time.
Second Interpretation: The sword is a higher-dimensional, large size construct that is just literally, physically cutting into the universe.
This is what happens if you just take everything at face value, I guess. Void is bigger than the universe. His sword is bigger than the universe. He is cutting into the universe from Hyperspace, with the sword that is bigger than the universe, slicing across space-time. Could also just be considered
Infinite speed I guess. This would also give Saitama
Infinite or
Immeasurable LS.