the point of diminishing returns

1. In economics, the point at which a yield rate (i.e., of profits, production, benefits, etc.) no longer grows in proportion to the amount of production, investment, time, etc., that is added. To combat high volumes of customers, the restaurant hired a large surplus of wait staff and cooks. But it seems to have reached the point of diminishing returns, as the overcrowded staff is far less efficient and costs the restaurant more in wages than it is earning. Marine biologists warn that fish populations in the oceans are long past the point of diminishing returns, with the amount of fish caught continually decreasing even as more fleets set out into the waters each season.

2. By extension, the point at which the quality or quantity of output (e.g., of a product, project, organization, etc.) stops increasing in proportion to the additional amount of time, money, skill, effort, etc., being spent. Unfortunately, the show's charm has not lasted. Despite increasingly zany plots and more characters than you can keep track of, it seems to have reached the point of diminishing returns. Schools have continued to pile more and more homework on students to catch up to standardized test requirements, going well past the point of diminishing returns. Students are burning out because of the huge workload, and their academic performance is stalling anyway.