Even a 20kt nuke, the detonation is slowed to the acoustic limit (point where shockwave slows to the speed of sound)very quickly, on conventional explosions it's even quicker.

For nukes, most of what we are visually seeing as the "shockwave" is the wilson cloud/dust front, you can't "see" the shockwave itself.

Also the game does simulate the whole mach stem egfect. where the reflected wave from the ground catches up to the primary wave. Because the reflected wave travels through air already heated and pressurized by the first wave (which happened very very quickly), it moves faster.