The article proposes an opportunity cost model to explain the subjective experience of mental effort and its impact on task performance. The authors argue that mental effort is not a depletion of a physical resource but rather a conscious measurement of the cost, particularly the opportunity cost, of continuing a task. They suggest that certain computational mechanisms, especially those associated with executive function, can only be deployed for a limited number of tasks simultaneously. The deployment of these mechanisms carries an opportunity cost, which is the felt output of cost/benefit computations. This subjective experience of effort motivates the reallocation of computational processes to more valuable tasks, leading to performance reductions. The authors review existing literature on mental effort and performance, highlighting the limitations of previous models and proposing a computational framework to explain these phenomena. They also discuss the role of motivation and the potential for empirical tests to distinguish their model from alternative explanations.The article proposes an opportunity cost model to explain the subjective experience of mental effort and its impact on task performance. The authors argue that mental effort is not a depletion of a physical resource but rather a conscious measurement of the cost, particularly the opportunity cost, of continuing a task. They suggest that certain computational mechanisms, especially those associated with executive function, can only be deployed for a limited number of tasks simultaneously. The deployment of these mechanisms carries an opportunity cost, which is the felt output of cost/benefit computations. This subjective experience of effort motivates the reallocation of computational processes to more valuable tasks, leading to performance reductions. The authors review existing literature on mental effort and performance, highlighting the limitations of previous models and proposing a computational framework to explain these phenomena. They also discuss the role of motivation and the potential for empirical tests to distinguish their model from alternative explanations.