I googled this before. I saw a credible treatise for #1 and #4. to the tune of about a 10% change for me.
If we take just the two big ticket items from the above list, Higher Air Density, and Less Aero Shape (drag) we have forces at work (and some numbers to back them up) that can account for 15 to 30 percent greater effort to maintain the same speed, or 15 to 30 percent reduction in speed.
So using these numbers, a 20mph rider gets reduced to 14 to 17mph (30% and 15% reduction respectively). The 17mph rider gets knocked down to 14 or even 12 mph.
We have therefore accounted for typical speed reductions reported by die hard ICEBIKERS, without having to resort to use of those factors of which we are unsure.
When you add to this an allowance for lower tire pressure, the added rolling resistance of studs, the increased wind in winter, and the effort needed to plow through snow or slush you will see that there is ample reason that we are indeed Slower In Winter.