the law of diminishing returns

1. In economics, the law that a yield rate (i.e., of profits, production, benefits, etc.) will eventually fail to grow in proportion to the amount of investment, skill, time, or effort that is added. The restaurant, to combat high volumes of customers, hired a large surplus of wait staff and cooks. The law of diminishing returns kicked in, however, as the overcrowded staff was far less efficient and eventually cost the restaurant more in wages than it was earning.

2. By extension, the idea that any output or results (e.g., of a product, project, organization, etc.) will eventually begin failing to increase in proportion to additional time, money, skill, or effort being spent. The show's charm has not lasted, even as more zany plots and characters keep being added. That's just the law of diminishing returns, unfortunately. Schools feel like they have to pile on homework so students meet increasingly high targets for standardized tests, but then you rub up against the law of diminishing returns, because students can only handle so much work before they begin to burn out and perform poorly anyway.

the law of diminishing returns

used to refer to the point at which the level of profits or benefits to be gained is reduced to less than the amount of money or energy invested.

This expression originated in the early 19th century with reference to the profits from agriculture.