I think that most folks here are using something in the order of the 55,000 btu turkey burners. They will easily bring 3-4 gallons to a pretty quick boil.
Most stovetop ng burners have a high range of 15,000 - 16,000 Btu and my Charbroil propane grill has a side burner rated at approx. the same. Both can boil that much water within 20-30 minutes. The turkey cooker is probably your best bet for the speed you seek and afterwards it can make a mean deep fried turkey to boot.