I'm slower in cold even after accounting for bulky or loose clothes (with or without), denser air, lower humidity, wind, and the time it takes to warm up. All of these are factors, but even eliminating them I'm still slower. I think that means there must be physiological reasons. You see on the web (so it must be true ) that thermal stress impacts performance by about 10% but that seems a little glib to me.

I think it's due to a combination of several reasons. I'm sure I'm oversimplifying, but:

1. Body temperature is normally related to our circadian rhythm, so if the cold lowers our body temperature it will make us sluggish until we get warmed up.

2. I think that glycogen conversion is slower at lower temperatures and we instead rely relatively more on fatty acid oxidation, so it feels like we have less energy available. I could be way off base on that though.