Growing pains. Not really unlike how it used to be hard to start cars in cold weather- newer cars have heaters built into the battery, since its basically zero-cost, and enough insulation that they can get up to temp quite quickly. Heating the cabin is a bigger issue, since it's pretty hard to reduce what is a significant drain.

4 miles/hour is very, very slow. It's the maximum output of a standard wall charger. Maybe you were at a level 1 charger?