Lots of factors contribute to slower riding when it's cold and I'm surprised you only see a drop of 2 k/hr. They include:
1) denser air when it's cold - therefore greater air resistance
2) more clothing which restricts the movement of your legs and also adds air resistance
3) colder air in your lungs results in slower exchange of oxygen into your blood
4) body uses more energy to stay warm so less is available for leg muscles
5) more mechanical resistance when cold due to thicker grease/oil in bearings and less flexible rubber tires.