Well your blood is thicker in cold weather, muscles are tense and you (and everyone else) is generally uncomfortable in cold weather.
Some of it is physiological and some is psychological. I think everyone is more sluggish in cold weather. I add at least an extra 5 -10 minutes to any journey in the cold.
"Things look different from the seat of a bike carrying a sleeping bag with a cold beer tucked inside." ~Jim Malusa
2009 Trek 520-Brooks B-17 Special in Antique Brown
2010 Surly Long Haul Trucker-Brooks B-17 Standard in Black
1983 Fuji Espree Single Speed-Brooks B17 British Racing Green