I was watching a SciFi tv show where large objects had an outer speed limit of 18000 kph and that got me wondering what things in everyday life are faster than even 500 kph.
I know bullets can be fast, but they are not exactly everyday life (at least in my life).
I included mass for obvious relativistic reasons.
It seems that depending on the type of glass and the direction of the waves (longitudinal, shear, or Extensional) the speed of sound in glass can be between 2300-6000 m/s
Longitudinal is the type we normally think of though, and that is between 3900-5600 m/s. Which is still much more variation than I was expecting.
The speed of sound in air is around 340 m/s depending on temperature.
So if the op is correct about the speed, then it seems the cracks propagate slower than the speed of sound in glass.
https://www.engineeringtoolbox.com/sound-speed-solids-d_713.html