I believe you will be happier with the fast response for lighting control, I know I have been. The difference was quite noticeable in my application. I have not noticed any issues with false alarms but for lighting, a false alarm would not likely be a big deal anyway.
If you had a situation that required the very fastest response possible, I would expect the highest performance would be obtained from a zone on the main M1 board set for fast response and an output (if used) also coming off the main board. The reason for this would be that the communication with this I/O would not be over the RS485 bus. I have not been using the main boards I/O (and I have not done any comparision testing) but I would expect faster results from this approach. If this is not correct then hopefully Spanky or others will chime in. I definately would not want to mislead anyone.
There might be a downside for security applications:
In my experience the Elk's performance and dependablity has been rock solid and I have not noticed any issues with false triggers of a zone. I think it would be more likely that a false trigger would come from a connected device or installation/wiring issues than from the Elk itself, but the small delay in the triggering of a zone might help under some conditions. If I was doing straight security, I would probably leave the delay in hopes that it would reduce the likelyhood of a false trigger. So far I have only used my Elk for home automation so I hope some people with more security experience will chime in on this issue.
In reality, there are two zone loop response times referred to as Fast Loop Response and Slow Loop Response. When the zone definition is setup in the M1, either loop response time can be used. The default is slow loop response. You can set any time you want for either loop response times in the Global Setup Section of the M1 programming. A fast loop response could be set as a slower time than the slow loop response. The range is from 20 milliseconds to 2.55 seconds in 10 millisecond increments. The faster the loop response time is set to, the more chance of a false trigger. The zone loop must remain violated for the entire loop response time to be considered as a zone violation.
I changed the door that I use the most (the door between the garage and family room) to a fast loop response. This zone does have the chime enabled, and I also use that zone to trigger the garage lights. The only thing different that I have noticied is that sometimes when closing the door, if I didn't use my man strength and close it all the way, and perhaps the door ever so lightly opens again just as I realize I didn't get it closed, the chime will sound again. When I had it set to the normal response time, I had never seen this, unless the door completely reopened and I wasn't there to shut it just as fast as it had opened. I don't view this as annoying, or problematic, or anything. I am just documenting what I have seen as the only difference. I do like the lights coming on in the garage a few milliseconds earlier. You'd be surprised that you really do notice such things. And that's all I have to say about that.