The reverb, much like Shimano brakes, has a known issue when operating in the cold. The oil inside simply thickens too much to properly flow through the lever and hose. The oil flow through the damper also gets constricted. I don't think there's really a solution because switching to a thinner oil might cause issues in the summertime, such as the post rebounding too quickly and damaging internal components. Of course, you could switch to a different dropper with a more modern design that does not suffer from that issue.
Do NOT pull up on the reverb when it is compressed, even if the lever is being pushed. You will suck air past the seals and cause the infamous "Reverb squish" (where the post is no longer capable of staying rigid thanks to air being mixed in with the oil, and you basically have a very expensive suspension seatpost. Cannot be fixed with Viagra.)
Edit: If your particular model has it, try turning the speed adjuster to full open/fastest. That might help.