Is a theory in economics predicting that In the short run, when factors such as capital, technology is fixed, if we increase a variable input after reaching the optimal production point, every additional input will result in a smaller increase in total output, as the efficiency is limited by the other production factors.

Before optimal level of production is reached, specialization effect happen
,workers are practicing division of labor, each worker is assigned with a specific task which match their ability with appropriate working space, variety of tools. At this stage, with workers being added, it becomes increasingly possible to split up tasks to perform particular skills where their speed of operation can be increased and spoilage reduced. The marginal product per labor is improved eventually. However, with the technology, machines remain the same, the marginal productivity cannot increase permanently which lead to congestion effect when additional worker contributes fewer to production process, causing the total output to go increase at a decreasing rate.
Congestion Effect: the kitchen then becomes more crowded, workers have to share machines tasks are not fairly assigned among them. Then, they start to get in the others' way, the conflict arises resulting in their concentration and effort reduce significantly. And hiring more workers even cause the output decrease after reaching this point.
→ Therefore, if the company wants to postpone the law and exist sufficiently in the long run, they should invest more on technology or expand working space rather than focusing on hiring more labor only. (despite buying more machine will increase the production cost)

So how diminishing return affect cost? My teammate will introduce it to u