I think Justin is on to the answer. The loop back is apparently too strenuous a test of the 8SA (or Insteon traffic control??).
When I asynchronously change the input externally (i.e. not from the loop back), the ISY sees the change immediately. Even better, the program I set up with Input 1 as the trigger changes state immediately as well.
So it was just my test setup that caused the problem. Sorry to send everyone on a wild goose chase.
I’ll still work on getting my other two Access points installed in good locations.