I can honestly say that I'm writing to you with warm feet from our cob home. The temperature right now is below zero and we've recently had -5ºC with no issues.
Below zero temperatures can be destructive if you are trying to build with cob (think frost heave) but for living, there's no problem.
A wooden house (well, really, plastic with a bit of wood) will warm up faster because it doesn't absorb any of the heat, but it will also get instantly cold once the heat source has stopped.
Cob buildings will take longer to heat up if you're starting from the outside temperature, but if you're getting up in the morning and starting a wood stove (and had a warm house the day before), it takes a few minutes to heat up the house.
The big bonus is that cob houses take much longer to cool down once the heat source has stopped. The air in my little house gets down to about 16ºC at this time of year. 30 minutes later and we're in the 20ºs. A wooden house would be close to 5ºC unless the fire was kept going all night or you're on the grid with a central heating system.
Our bigger house will feature a rocket stove with in-floor and bench heating. These parts of the house will be warm for days after the fire goes out! When you heat furniture and floors, you don't need to heat airspace, which dramatically reduces the need for a heat source. Think about how all that warm air just goes out the window/door.
Of course, all my descriptions are based on sealed houses. Drafty houses are even worse, cob or plastic, at retaining warmth. The problem with sealing plastic houses is that they then create toxic offgases which create sick building syndrome. Cob breathes even when fully sealed and we don't use any toxic materials.
For much colder climates, I recommend thicker cob walls, 18" minimum. But I don't personally think adding materials to insulate cob walls are needed or outweigh the time/cost that they induce.
Cob is a thermal mass/thermal battery and is radically different than conventional building materials. There is no mass in conventional buildings so they need to insulate to try to retain the heat. But with cob and any other type of thermal battery, there is no need. Cob absorbs the heat when the air is hotter than the walls, and it releases the heat when the walls are warmer than the air.
That's what makes cob such a great natural air conditioner in the summer, too. Our cob buildings are always the coolest places in the heat of summer. Even when the daytime temperatures get over 30 degrees Celsius and there is no wind, I sleep like a baby all night long.
Winter or Summer or anytime in between, I'll take a cob house over any other and save energy, effort, and live with much more ease...thanks for asking!