Around here (minneapolis) the snow is cleared on the commuter paths before the roads, Many mornings for me the worst parts is getting to and from the commuter path.
As far as the original question, I've found that my deep winter commute takes about 20% longer on average than my summer commute. Riding studded tires, slippery conditions, and wearing a lot of clothes in dense cold air, it just takes a bit longer. I simply plan for it.