I think it is a mix of things: extra weight of clothes, increased friction between the layers of clothes over your knees, and the need to carry extra clothes for emergencies. Theoretically, your bikes bearings should warm fairly quickly and I would expect them to stay warm (think about riding your bike thru slush at the freezing point, the ice accumulates on the cogs you aren't using and your chain stays ice-free). Even your tires will warm up in a short distance, the difference there is studs and wider tires with tread that is being engaged. Winter biking is riding on a beach, uphill, with MTB tires. Why wouldn't it be slower?