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.