The peasantry worked fewer hours because of the nature of production. Capitalist commodity production seeks to maximize profits, and does so by ensuring wages are regulated by cost of subsistence and replacement. Feudalism operated differently, peasants worked for themselves and produced essentially rent for their feudal lords, and thus did not have maximized working hours.

Question 8 : In what way does the proletarian differ from the serf?

Answer : The serf enjoys the possession and use of an instrument of production, a piece of land, in exchange for which he hands over a part of his product or performs labour. The proletarian works with the instruments of production of another for the account of this other, in exchange for a part of the product. The serf gives up, the proletarian receives. The serf has an assured existence, the proletarian has not. The serf is outside competition, the proletarian is in it. The serf frees himself either by running away to the town and there becoming a handicraftsman or by giving his landlord money instead of labour and products, thereby becoming a free tenant; or by driving his feudal lord away and himself becoming a proprietor, in short, by entering in one way or another into the owning class and into competition. The proletarian frees himself by abolishing competition, private property and all class differences.

-The Principles of Communism

This isn't to say the serfs had it materially better, but that their mode of production was different. Their low level of technological development stood in the way of great progress, and we cannot return back to such a model, but instead should progress onward to Socialism now that Capitalism has largely run its course and centralized most modern industries for central planning and public ownership.

Marxism is useful for understanding this phenomena, I made an intro to Marxism reading list if any of you are interested.