Human task performance elicits diverse subjective metacognitive experiences, such as boredom, effort, fatigue and frustration, which are thought to play important roles in the monitoring and regulation of cognitive processes. Yet, their specific contributions to task performance remain poorly understood. Therefore, we investigated temporal dynamics underlying these metacognitive experiences and the cognitive processes supporting task performance. We used a time-on-task design using a conflict Flanker task, and analyzed the data using a multi-methodological approach involving behavioral, model-based, subjective, and neural measures. Our results show that the temporal dynamics supporting task performance can be understood as a rational attempt to optimize behavior and that distinct metacognitive experiences track different aspects of this rational endeavor. These findings suggest that metacognitive experiences act as tools for humans to gain insights into the optimality of their cognitive performance.