More layers, which you are carrying as weight all over. Dark so more cautious riding. Generally windier and wetter.

It was a mild november and my avg speeds were still decent, as soon as December hit my speeds have plummeted through the floor. But was on a club ride on saturday and the pace was fairly high as conditions were good. It just depends.

Wouldnt worry about it too much. Commuting this time of year is just about getting there and getting home again.

Winter plumage innit.