Pete writes:

So, my questions are; how long should I expect the heater to be
running before I can tell there is heated water? I've waited a few
minutes. Does it take a long time for the heat to penetrate the water
headers in the heater?

I hope you don't have some naive idea about pool heating.

Your small unit only delivers 80,000 BTU/hour. Let's say you're piping 50
gallons a minute. That's 50*60 = 3000 gallons per hour. 3000 gallons
weighs about 25,000 lbs. One BTU is one degree F per pound of water, so
the the temperature differential of your heated water will only be
80,000/25,000 = 3.2 degrees F above the pool inlet, hardly perceptible.
It's not going to feel like your household hot water is running into your
pool.

You will bankrupt yourself buying gas to heat your pool. Did your "good
pool dealer" tell you that?