Maybe it was the impending call of nature that quckened your pace.
Serously though, colder weather most assuredly improves your results. In addition to the overheating / core temp issues, colder air is denser, it carries more oxgen in it. For a given lung capacity, your VO2 max goes up as the temp drops. Higer VO2 max means more ATP avaialbility, which which means greater aerobic burn in your muscles yadda yadda yadda, which means you can run faster.
(Once warmed up, Naturaly Aspirated cars run better in the cold too. That's why turbos have an intercooler)
-dave