Sorry for the confusion. The six second delay is only the startup time (after a power up) for the EZIO2X4, EZIO4O, and EZIO8T. The startup on the 8SA is 2 seconds.
More directly to your question, the unit responds to a change on an input within .25 seconds including an Insteon message being sent. On a worst-case basis, assuming multiple units linked on that input, and other traffic, the response is about .75 seconds. This assumes direct linking, not going trhough another controller that is rebroadcasting the input change event.
I hope this answers your question.