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This is a thermodynamics issue: the heat capacity of steel is about 460 times the heat capacity of an equal mass of air -- and there's not a large mass of air in an oven, and each rack weighs a couple of kilograms. So each additional rack is a major factor in the preheat time.

The rapid heating mode turns on the broiler and burner and convection fan to get the air temperature up as fast as possible. To aid the marketing number, they tell you to only put in one rack. If the marketing department could convince engineering to say that no racks should be put in, the time to pre-heat would be even faster.

If you are depending on a stable temperature in your oven, adding thermal mass will slow down the preheat but smooth out fluctuations. Broil a steak? Use the pre-heat and one rack. Baking a cake? Assume that the rapid preheat is only a little faster than normal and put more thermal capacity in your oven.