The footage is hugely slowed down, hence the deep pitch of the rocket motor sounds (they are usually very 'hissy' at this size), and the slow speed of the sparks coming from the motors. Also if you listen closely when the rocket is descending and deploying parachutes you can hear the greatly-slowed-down sound of a DSLR shutter actuating repeatedly (ka-thunk-beep) as photos are taken of the descent.
To avoid disturbing others in the same room, I watched this at first with sound entirely off. Watching it again with the benefit of sound it's blindingly obvious that this is slowed down significantly... Facepalm time.
That's one of his design goals, most model rockets are unrealistically fast compared to the real thing