This topic has been discussed before. But I think I've thought of a new reason for the bike to be slower in the extreme cold.

In previous threads, I've heard mention: air density, tire rubber (less pliable = more rolling resistance), hub grease (mentioned that one myself a few times), and the extra layers, all add up to slow you down.

but I came up with a new possiblity while riding yesterday (-10F) that I haven't heard or read before. At least it's new to me, I haven't heard or read it mentioned before

Shrinkage. yes the same thing that happens to men in cold water, can happen to your bike parts.
Have you ever adjusted your cones? just a degree or two in either direction can make the difference between loosey goosey, just right, or way too tight. What happens when your axle gets cold? it shrinks, contracts, gets shorter, trunkates, thus compressing your cones & races around your bearings, making your cones too tight and slowing you down.

Yes i know, your bearings are shrinking too, but They're probably not the same alloy, so they're shrinking at different rates.

this might also hold true for cartridge bearings, Inner rings contracting on bearings, but probably not to the same extent, since there isn't as much metal in that direction of contraction.

It's my hypothesis.
I'm sticking to it.