Sterno takes around 20 minutes to boil 2 cups of water because it doesn't burn as hot as some other fuels.
Other alcohol based cooking stoves can boil 2 cups of water in just 8 minutes which is half the amount of time needed for the Sterno to boil the same amount of water.
Sterno can boil water but Sterno is more suited for keeping liquids warm or food warm than it is for boiling water although you can boil water with Sterno but it takes a bit longer to get the amount of heat out of it that is required to boil water.