Why must batter made with single-acting baking powder be baked quickly?

Batter made with single-acting baking powder must be baked immediately because the chemical reaction that produces carbon dioxide begins as soon as the powder is moistened at room temperature.12

Unlike double-acting baking powder, which remains relatively inert until the heat of the oven triggers a second release of gas, single-acting varieties release their leavening gas during the mixing and makeup stages. If the batter is not baked quickly, the gas escapes into the atmosphere rather than becoming trapped in the batter, which would result in a loss of leavening power and a failure of the baked good to rise properly.13