Problem solved; thanks for the reply though.
With Event Viewer open, I ran the program to set the reduced on-level, and was surprised to see ISY attempting to write to several different devices, with the bathroom SwitchLinc being last of all, and taking about a minute-and-a-half, which explains the delay.
On examining the devices, I discovered that they were all wireless devices which had updates pending, and a closer look showed, in the Administrative Console, that the button to automatically write changes to wireless devices was enabled. I normally keep that button disabled, and it seems that when ISY reboots, it enables that button--something to be aware of in the future.
With that button now disabled, and again running the program, response was immediate, and all is back to normal.
Algorithm's post in Strange issue with Adjust Scene command. was marked as the answer