I did a quick experiment with my mapp torch to determine what was the cause of my scorched flux. First, I brushed a bit of flux on a bare tube and slowly brought it up to temperature and checked by flowing a small bit of silver on the surface of the tube. Then I held the area at that temperature for a couple of minutes or more, being careful not to bring any of the tubing past a dull cherry red by flicking the torch away. I deliberately scorched a small area just out of interest, then turned the torch off. Compared to the scorched area, the rest of the flux retained a glassy look despite being held at temperature for a long time.
Just now, I took a practice lug I had brazed to the end of a tube, gave it a quick clean, fluxed it and got the torch on it. Within 20-25 seconds the tube just beyond the lug was cherry red (heating too quickly relative to the lug itself due to my poor torch technique!) and by 30 seconds I was flowing the silver in the joint. Maybe not the most representative test as I was just re-melting pre-existing filler, but that seems to be pretty quick to me. I really do have to be careful with not locally overheating the flux (which I even did on this very brief test).
Loved the youtube clip! My mapp torch has a smaller flame than the one they use...
I'm not trying to say everything people say about mapp is wrong, just curious to understand what the differences really are. I had one brief experience with oxy acetylene many years ago and remember it putting out much more heat! I just don't have enough experience to make a meaningful comparison.
When I get some more practice lugs I'm gonna film my brazing attempt and post it for interest.