Material stress can propagate at faster than the speed of sound (in air). If it's a concrete structure, the stresses can be applied at the speed of sound in concrete (whatever that is). Collapse speed would be carried out by the acceleration of gravity (1G).
Following Occam's Razor, the buildings were most likely destroyed the way we observed them; structural failure following intense heat from jet fuel burning.
How fast can a Prince Rupert Drop structurally fail?