I definitely ride slower in winter too but I always assumed that was due to my body having a harder time in the cold rather than air density changes. That seem

I definitely ride slower in winter too but I always assumed that was due to my body having a harder time in the cold rather than air density changes. That seems to be born out by using better (read more expensive) winter riding gear. I naturally run a bit cold so spending on low temperature tights, base layer and jersey/jacket has made a massive difference for me.