The propagation delay time (tPD) is measured as the time between an input edge to an output change. This time is composed of two parts: an input deglitcher and output slewing delay. The input deglitcher prevents noise on the input pins from affecting the output state.
The gate drive slew rate also contributes to the delay time. For the output to change state during normal operation, one FET must first be turned off. The FET gate is ramped down according to the IDRIVE resistor selection, and the observed propagation delay ends when the FET gate falls below the threshold voltage.