

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










Serious answer:
From wiki: “Fascism is characterized by support for a dictatorial leader, centralized autocracy, militarism, forcible suppression of opposition, belief in a natural social hierarchy, subordination of individual interests for the perceived interest of the nation or race, and strong regimentation of society and the economy.”
I’m not a historian or anything, but I would say that until trump we haven’t really had anyone you could call a dictator. And saying we had a strong regimentation of society and the economy seems like a stretch with the exception of wartime. You could make a good argument for ww2 era USA under FDR though that also came on the back of the new deal and a relatively progressive thrust in US politics (of course many groups of people were still oppressed and marginalized though), and possibly some parts of US involvement in Vietnam. But I think realistically there are better descriptions of the US government during those times.
This comment is intended not to discount the historic atrocities and abject failings of the US government, but rather to maintain the power of the word fascism and to assert that things are in fact different at this moment, even if they’ve been bad for a lot if people for a long time.