Cooler air does slow you down. Anybody who has done their fare share of riding knows this by actual experience. I average about 1 mph less in the winter. Part of this is the extra clothing and lack of motivation to pedal hard in harsh weather along slippery surfaces. And then there is the reduced tire pressure and soggy leaves and wet pavement to add to the mix.
If you have ever bombed down a long decent and hit a patch of cooler air you will notice a dv/dt.
And of course the humidity is a lot higher in the winter time.
But these are just my viewpoints, put forth to help you with your holiday depression.