What factor on the trail has a great impact on the rate of travel and the safety of the team?

The ice is the real problem. . It takes 6 days for them to travel thirty miles. Occasionally they break through the ice, and the dogs have to be thawed by a fire. Perrault is relentless. He cannot be daunted, and wants to clear the 400 miles to Dawson as soon as possible. Several times the sled is almost dragged under the ice, but each time Perrault manages to get them over the next crest. Also, the dogs' feet were not meant for ice which makes their paws sore.