Heat of combustion is a measure of the amount of heat energy released when a given fuel is burned.  The larger the number the more heat energy that is released in the burning process. The more heat energy released the faster a substance being heated will increase in temperature. It is similar to heating two pots of water on a stove using either methane or propane.  If one burner is set on high, while the other is on low, it will take the burner set on low longer to heat the water because less heat energy is being applied to the water.