I find that the main slowing factors are studded or heavier tires, winter clothes (boots, heavy pants, etc.), and the greater winds.

The main factor where the cold plays in that I know of is wind chill. Riding faster makes you feel more wind, so if you're cold there's a trade-off between pedaling harder to warm up (generating more heat) and going slower to lessen the wind (losing heat less slowly).

This is all assuming perfectly clear roads, if there's snow that'll slow me down. A lot...