The first harvest is the quickest because of the animals' post- revolution zeal.
Chapter three opens with a discussion of the animals' impressive work output. Orwell writes that they "toiled and sweated to get the hay in." This resulted in the rewarding of their first harvest. It was the quickest and "an even bigger success than they had hoped." The animals were excited about the farm after the revolution. They saw it as their own. They had run Jones off it and believed themselves to be the masters of their own world. As a result, their work ethic was at a high level.
Orwell suggests that the enthusiasm with which they performed their labor was a direct consequence of the revolution. Their excitement facilitated their significant production. They referred to one another as "Comrade" and the text notes that they worked together:
There was no wastage whatever; the hens and ducks with their sharp eyes had gathered up the very last stalk. And not an animal on the farm had stolen so much as a mouthful.
The pigs did not toil, as they were directing the labor. However, the harmonious nature of the animals' efforts was a critical element in the speed and output of the first harvest.