Bottom line, even if you start at 50F and want to reach a boil in 6 gallons, it only takes 8,000 BTUs of heat to do this.
Your burner is rated at what?
One pound of LP is about 22,000 BTUs
It should only take 1 pound of LP to reach a boil in 6 gallons from 50F...
This is a mystery, but gas burners are more eff. at lower power settings.