To start off a shock or blast front in this case is caused by a shock wave traveling through the air faster than the speed of sound (*https://blastinjuryresearch.amedd.army.mil/index.cfm/blast_injury_101/science_of_blast*.) the shock wave is apparent by the low pressure followed by the blast front causing the water vapor to condense in an effect known as a Wilson Cloud. therefore the explosion definitely cause a pressure wave that moved faster than the speed of sound. to calculate that speed using the video, you would have to know the distances of nearby objects to the explosion, and use the framerate as a reference for time.(Distance/Time)
I do not know how I would find that information, but there is another way to find out by the use of, of course, math. the formula can be found in detail here: (https://aip.scitation.org/doi/10.1063/1.1667908) You only need to know a few things: 1 the amount of gaseous molecules per gram(as well as their weight) 2 the average weight of the gases 3 the amount of chemical energy in the reaction. Unfortunately here at 4 we have a problem the: initial density. (The above formula only measures detonation pressure not velocity and pressure. However pressure and velocity are intrinsically linked, but we run into the same problem of initial density. If you would like to know more about how you calculate many factors involving explosives you can find more here: https://www.sciencedirect.com/topics/chemical-engineering/detonation)
the problem is that it's almost impossible to know the density because the explosion ripped open the tanks of fuel and oxidizer. That caused the two to mix to some extent before the ignition source reached the containers. they then mixed more as the pressure waves propagated through the mixture. All in all it's safe to say that the pressure of the tanks is not a good way to estimate the density, as they vented out their contents to the atmosphere at an unknown rate.
That leaves us no real way of knowing the speed of the shock front without knowing the exact dimensions and distances of nearby objects along with a camera looking from the correct angle with a high framerate. I'll give you a pretty good guestimate of around *4400-6000 m/s which comes from an old study that tested the detonation speed of liquid Methalox.
*DETONATION OF LIQUID OXYGEN-LIQUID METHANE SOLUTIONS
A. V. Grosse, A. D. Kirshenbaum, and A. G. Streng
Journal of the American Chemical Society 1957 79 (23), 6341-6342
EDIT: As for the comments saying this was a deflagration not an explosion, the oxford dictionary defines deflagration as:
combustion which propagates through a gas or across the surface of an explosive at subsonic speeds, driven by the transfer of heat.
There was a very clear shock front, and while it is obvious not ALL of the methane and oxygen detonated a good amount did. The shockwave is undeniable proof of that.