The cold cast iron has a lot more thermal mass than cold air in the oven. If you think about it, heating up the inside of the oven is all about getting the inside surfaces and racks sufficiently hot to offset hot air venting. The oven will have to transfer heat into the cast iron to effectively turn it from a cold thermal battery into a warm thermal battery. That will likely add time to the preheat, because it is more material to heat up than just the surfaces and racks.

en.m.wikipedia.org/wiki/Thermal_mass

The contents of the oven have a certain heat capacity, which means you need a certain amount of energy to raise their temperature a certain amount.

If your pan has a higher heat capacity than the air it replaces, putting the pan in would increase the total heat capacity in your oven. That means more energy is needed to heat it up.

Now your oven dumps heat energy into it’s contents at a certain rate. This rate will not be affected by the pan, it only depends on a) the rate at which the heating elements release heat energy and b) the rate at which heat escapes the oven.

So adding the pan means more energy is needed to reach the same temperature, but the oven still supplies energy at the same rate, so it takes longer.