I noticed on one of my vacations where I took my Esbit stove and fuel and on a particularly windy day, it took a long time to boil the water. I was using some foil as a windscreen and my pot and stove were on a rock with a very irregular surface, so the wind I am sure played havoc with the heat getting transferred to the pot of water, since I had quicker boiling times on calmer days so I can imagine with the lower heat output of alcohol, that with no windscreen it would have been impossible to get that large amount of water to boil with a small amount of fuel.