I bike commute year-round and often ride in below-freezing temps, but it usually doesn't get much colder than about 15 F here. I actually enjoy riding in the cold -- assuming it's dry and I'm wearing adequate gear -- and prefer it to hot humid weather. However, it's definitely more difficult and slower. Cold air is much denser, causing more wind resistance. Your muscles also take a lot longer to warm up and become more efficient. My average speeds are typically 1-2 mph slower on cold days.