In the summer I ride with a t-shirt and no gloves. Therefore I can not only see, but operate the countdown on my watch, seeing if I can match the target of 13 minutes to the mid point, and then 13 minutes to my destination. So when I get on the quieter bits, instead of relaxing I am reminded of the passage of time, and the need to get a move on to beat the clock.
In winter I wear a long sleeved top and gloves. These stop me seeing the display, and makes using the buttons much more difficult. Hence I am not chasing time and take a bit longer.
So yes, I do commute slower in the winter than the summer....