At the beginning of the video the blades seem to be spinning at about 2RPS, but after the explosion they're spinning much faster. I presume the former was an artifact of the camera's frame rate.
They normally have high-speed cameras running; that short video is a mish-mash of super-slow-mo and regular speed footage. There are much longer versions on YT, e.g. this one [0] which covers it in more detail (watch the people in the room jump when it blows - despite being a fair distance away).
