during the period those were published i believe that most in the rife forum were using the SG-1A sending device. the dwell times are sometimes very short because the metal dumps or other toxin release, from pathogens like lyme, can be severve as you probably know.
can't really speak for Newport on this, but i expect that the short dwell time is a cautionary setting, so that those who are less experienced don't get carried away and inflict serious pain upon themselves with die off/toxin dump.