[MN Renovator]

180 seconds won't cool your ductwork down, it'll still be warm coming out. The delay is usually a setting for comfort to where the goal is to get the heat from the heat exchanger into the house but to stop it before it gets below a temperature where it will be creating a breeze cool enough to make you feel cold if you are in or near a vent that is blowing air. My own furnace is 75 seconds without an adjustment but when it shuts down I'm still getting air over 40 degrees warmer than the return air temperature which is room temperature. If 'cooling' the ductwork was an issue we would see there being problems with the low power constant fan applications in variable speed furnace systems.

Blower speeds have 2 main issues Temperature rise over the heat exchanger and electrical usage. You don't want the temp rise to be too low or you can risk condensing in the heat exchanger which can lead it to a short life. If your fan speed is set at a higher speed than the ductwork will allow, as in temp rise barely changing with the adjustment, you might be wasting energy by turning it up.

With my furnace at 75k BTU, its set to the med-lo blower tap by default and temp rise is 55f. The spec is 55-85f. If I raised it to the highest speed tap the temp rise drops to 50f and if I drop it to the lowest fan speed tap, 1/10HP the temperature hits over spec in about 5 minutes when I tested it so I immediately shut off the burner. The 1/8HP tap does the job with a standard lower end pleated filter. If I use one of the 1" 3M Ultimate filters that look like they have hundreds of tiny pleats the temp rise is 80f on the same tap which puts me at the high end of the range so I have to be careful about if that filter gets dirty. Instead of spending money on those filters or raising the blower tap, I just use a less restrictive filter which works out great and even when it looks a little gray the temp rise is still in check. At the moment since my furnace is running about 30 hours in December and so far only about an hour a day so far this month too I've used the 3M Ultimate filter with the higher blower tap because 3.4 amps(408) is all my new PSC motor uses(versus about twice that with the old one) and I get the filtering that my house really needs as there is quite a bit of dust floating around the bedroom from changing my clothes and ruffling of the bedsheets. You do get better natural gas fuel efficiency with a lower temp rise but you'll never get down to 10f temp rise unless you are pushing TONS of air which your furnace will never be capable of even if the ductwork is as big as the furnace can support at the high speed blower tap.

Reading the manual will tell you to use the blower setting that puts you within the temp rise range. If both M-Hi and Hi do the trick, its up to you if you want to try to get more out of your fuel or more out of your electricity. I usually go for more out of my fuel but at the moment my priority is to get more filtering which means more air flow and another 120 watts while running which will be an extra dollar if my furnace runs for 75 hours this January(9 kwh at 11c/kwh). Your blower will use more power since your unit is a 100k unit.