If I'm not mistaken, rolling resistance is a bit higher when it's cold out. I know I experienced the same thing commuting yesterday. I was quite a bit slower than I usually am, and the cold may have had something to do with it. Or maybe it's because I didn't get as much sleep as I should of the night before or because I haven't checked my tire pressure in over a week and they were low. Whatever the case was, the commutes definitely took longer.