Everyone rides slower in winter. Air is about 10% denser at freezing than in warm summer temps. You're wearing more clothes, that increases drag and decreases your mobility. You may be running fatter tires or studs, possibly at lower pressures. Around here it tends to also be windier, and the wind is almost always from the west which is a headwind on the way home (there's usually little wind in the morning, so I don't get a tailwind then as a bonus)

In summer my times are about 35 minutes if I'm pushing it, 39 or so if I'm just noodling along. In the winter on the same route, 43 minutes at best, as much as an hour if it's really bad (20-30 MPH headwind into driving snow).