In his post the other day about winter cycling, CycleDog noted that "you ride more slowly in the winter ... probably due to the extra clothing and the related drag."
ItsJustMe commented, "a surprising amount of the slowness in the winter is due to the air being more dense."
In addition to these factors, this article at IceBike also mentions stiffer grease, energy use in your body redirected to keep you warm, greater rolling resistance in the tires, and the discomfort of generating sweat in typical winter clothing.
The Real Reason
I have discovered the real reason for slower cold weather bicycling. During this morning's commute I wore tights and top and a fleece cap under my helmet. The clothing is snug but not confining so my motion was in no way restricted. I didn't wear a plastic shell -- everything I wore is very breathable, yet the midweight fabric was just thick enough to keep me warm. I rode my lightweight fixed-gear skinny-tire road bike in. The temperature was a comfortable 25° F (-5° C) during my 30 minute commute. I was all set to go fast.
Too cold to breath
But I went slow. I sprinted to 24 mph near the end of my ride to beat some cars to an intersection. The physical effort wasn't any more difficult than usual, but I felt like I was going to die. Why? Because it's too cold to breath hard. Sucking subfreezing air into my lungs hurts. Here it is three hours later and I'm still coughing up mucous and gunk from that sprint.
We go slow in the cold to avoid oxygen debt and hard breathing. So, like the CycleDog writes, take it easy, go slow, and protect your lungs.
I've already had 18*F temps this year in the mornings, typical is about 25*F. I've ridden at -10*F. I've definitely had days where I'm coughing all day due to a scraped-up throat from the cold. I've read the psolar air throughput is OK if you don't sprint. I am not a racer, I only maintain at most about a 16.5mph avg. Also I've found that just a 0.5mph drop in speed results in a cakewalk ride; averaging 15 is hardly sweating, 16 is some work, 17 is hard. So hopefully I can keep some level of speed up with this mask. I'll report probably on bikeforums.
I have a neck gaiter I picked up from MEC that does the trick quite nicely - it warms the air up just enough for it to be breathable, and it's loose enough that it doesn't interfere with breathing. It usually doesn't get colder than around -20C here in Nova Scotia, so I don't know what you'd need below that; it might be worth checking some of the bloggers in Calgary if it gets that cold.
Too cold to breathe? This must be another non-Norwegian problem (such as cold crown-jewels). :) Come on, guys! It has to be REALLY cold before you damage your lungs during hard exercise. Think about the cross-country skiers...