Preheat by all means. The other day I did 11 gph in my first hour with a 2x3 pan.

But the pan had some help. I had 4 buffet pans on turkey fryers going at the same time.
I only had 20 gal of sap to work with so I emptied those buffet pans pretty quick so didn't use much propane at all and never lost the boil.