Why does the rate of propagation matter? I presume that during the course of the explosion there are several sequential phases. If phase x emits some non-EMP but detectable signal, say an X-Ray burst and phase y emits the EMP, both of which propagate at the same rate, then you have time(y) - time(x) to respond to detecting x before the EMP from y reaches you.

Explosions, like all other macro scale phenomena are not instantaneous, they just appear so until an appropriately small time step is applied.