The primary reason I am slower in cold weather is that I simply don't want to exert myself to the same level and start sweating, and then get cold. Also, I don't think muscles work as efficiently as the temperature drops. Other reasons are that I am carrying extra weight in clothing and lights, and I use heavier tires in cold weather for better flat protection. Also, friction on moving parts increases, tires have increased rolling resistance, the air is denser at lower temperature, but I think these are less significant. Every little bit adds up to a noticeably slower ride.