Basically, the Endless fires a beam across the planet as Megido is falling, but before he lands. Given the general height Megido seems to be at, this feat likely takes place between 3 and 7 s.
Looking at this, we can see the beam crosses at least 1/7th of the planet, since it visibly moves over a bit under 1/3rd of the hemisphere. Since the Earth is 40,075 km in diameter, this means the beam traveled at least 5725 km. This, combined with the timeframe estimates based on Megido's freefall, gives us all we need to finish this calc.
v = d/t
- d = 5,725,000 m
- t = 7 s
5,725,000/7 = 817,900 m/s
v = d/t
- d = 5,725,000 m
- t = 3 s
5,725,000/3 = 1,908,000 m/s
Results
- The Endless Blasts Across the Earth (7 s) - 817,900 m/s or Mach 2403
- The Endless Blasts Across the Earth (3 s) - 1,908,000 m/s or Mach 5607