I always get slow in the winter due to the studded tires, heavier clothing, and dense air. I try to keep the best pace I can without getting winded because breathing real hard when the air is below 0 may not be too good for my lungs. My normal commute is 1 hour and in the winter it's 1 hour 15 minutes, so the difference is real but I allow more time and keep the enjoyment level of the ride up there.