With all the factors in winter, ice, snow, wind,heavy clothes etc etc. realistically expect about a 50% decrease.
My average speed in non-winter months in 16-18mph and in the winter I drop to about 8-9mph for an average.

10% decrease is unrealistic unless you only ride after the roads, trails have been plowed and there is no rain, snow,ice, wind etc.

The good thing is that after cycling in the snow for many months, when it is dry pavement you fly and are in great shape for the fall, summer etc.

Ice will slow you down (even with studded tires) so don't think about fast, think survival and balance and when the snow gets to be about 6" deep you really slow down...
You will spend a lot of energy just staying warm and sometimes the cold air takes your breath away(I have cycled at sub-zero temps with windchill bringing it down to -20, deep snow, ice....
You will question your sanity at such times but you get used to it...lol

You should seriously think about getting studded tires, you may get by if you are careful on ice, but if you are going fast and hit black ice you are going down, even with studded tires expect to go down.
Studded tires are cheaper than strains or broken bones.