It sounds as if you compared the time it takes to bring a pot of cold water to a boil on a stove with the time it takes to bring the same amount of cold water plus a layer of vegetable oil floating on top of it to a boil on the same stove.
Liquid water evaporates from the surface of the water in the first pot all the time. At higher temperatures, this evaporation rate is higher. Evaporation takes lots of energy (540 calories per gram), and cools down the liquid. This process competes with the rate at which heat is added from the hot stove.
If a layer of vegetable oil floats on top, it prevents the evaporation from happening, and so the water retains a larger fraction of the heat given to it by the stove and comes more quickly to a boil.
Water will boil more quickly if you put the lid on the pot. But the vegetable oil will still help out a bit here, because the evaporated water in the covered pot, while it does not escape, carrying off huge amounts of energy, some of it condenses on the lid and drips back in, cooling the liquid.
(published on 10/22/2007)