The windmill symbolizes a good life under the new leadership of Animal Farm. It stands for a society equipped with all the vital tools needed to reduce "necessary labour time" to a minimum and free up everyone's lives to pursue passionate passions, inside and outside of work.
Yet the revolution in machinery is not so straightforward as that. As Marx notes, the individual capitalists' interest in using machinery to make their commodities as cheaply as possible does not necessarily equate with a reduction in the amount of labor required to produce them. Indeed, if one capitalist's new technology allows them to undercut their competitors and capture a larger share of the market, it also means that other capitalists will quickly catch up and overtake them. This is a key reason that the rate of profit tends to fall once new technology becomes widely used: as more capitalists adopt the same technology, each's advantage becomes less significant.
What's more, machinery also has the potential to increase the speed of production. This is why it's not surprising that the U.S. leads the world in hours worked per week. The rapid development of labor-saving machinery allowed capitalists to boost productivity and expand the exploitation of urban space, pushing agricultural workers and peasants out of rural areas into cities.
The result is that, even though the revolution in machinery has made it possible to work shorter hours, the overall length of the working day has increased, as has the intensity and pace of labor. This is not a result of the technology itself, but rather of how the technology is deployed by the capitalists.
