I have tried both (i set up fast on controlling two lights as responders and set up fast off to adjust a scene).
Fast Off - adjusting a scene was very slow (a few seconds)
Fast On - controlling two other lights as responders was faster - close to a second - but no where near as fast as if they were directly linked.
Oddly both worked less than 100% of the time.
Even more oddly, i could see the light blinking on and off on the wall plug responder as if it was seeing Insteon data being transmitted, but the light status was not changed. I would then do fast on or fast off a few more times and eventually they would take.
So two issues: speed and reliability.
At the end of the day, the speed is not great (not instant) - is there way to program the responders to respond directly from the switchlinc Fast On, rather than having to go through the ISY and wait for the ISY to listen, decide and retransmit the command?