Just an idea: Check your tire pressures - they drop a bit as the temperature falls. This is not a big change, but it could affect your performance some.
I've not noticed any performance change with the cold. Typically winter temps are mid 30's in the morning, but we have had 3-4 days i the 20's. One morning dropped to 19! I do have a bit of a motivational challenge getting started, but once I am moving, the weather doesn't seem to matter (except for hail or sleet or anything else that hurts when it hits)