Psst! Wanna watch glass shatter at 28,500 frames a second? The Slow Mo Guys have you covered! The idea here is to see how fast a crack will move through glass, and you need a rather powerful camera to measure that. They have to do it more than once to get the technique right, but the speed of a crack turns out to be pretty darn fast. I wonder if thicker glass would crack at the same speed- but that would be another video.
According to this site, the speed of sound in glass is 4540 m/s. So, it breaks at a third of the speed of sound. I wonder if other brittle materials have the same behavior.