There is usually a diagram explaining what is measured.
It is usually the time measured between 50% points of the input and output waveform.
The maximum switching, or toggle, or clock, frequency of a device is usually on the order of the propagation delay (i.e., for a 32ns delay, ~32MHz). But it can be greater or lesser, depending on internal design.
The very fastest logic is little slower than the speed of light across the package: an internal propagation delay <100ps, yet the delay as measured at the pins is 300ps or more -- because the signal has to propagate along the pins to get there, first! This is one of the more extreme cases, where the clock frequency can be much higher (10s GHz?) than the propagation delay, simply because the propagation delay, as measured at any useful location, is several wavelengths already. (Needless to say, design of digital circuits at those frequencies requires considerable thought in matching up the delays.)
Tim