INTRODUCTION: The relationship between cognitive tasks and physical performance has garnered significant attention, with evidence suggesting that cognitive effort before exercise may impair physical performance. However, recent findings challenge the robustness of this effect, necessitating a reassessment of the mechanisms linking cognitive load to physical performance. This study introduces a novel approach to address methodological limitations, emphasizing individualized cognitive task difficulty and duration. Using techniques such as Temporal Experience Tracing (TET) and psychophysiological monitoring, we explore the dynamics between cognitive effort, subjective states, and physical performance. METHODS: In a pre-registered, randomized, within-participant design experiment, 21 recreational athletes completed a running time to exhaustion test at 90% of their maximal aerobic speed after performing a cognitive task until failure or watching a self-selected documentary. Pupillometry and six subjective dimensions were measured with the TET during task performance. RESULTS: We found that 1) subjective changes during effortful tasks are not limited to a single experience, such as mental fatigue or boredom, but can be grouped into distinct patterns; 2) the individualized and demanding cognitive task, completed before exercise did not impair subsequent physical performance; 3) pupil size reliably reflected cognitive load and is partially related to changes in subjective states, while fixation on the stimulus decreased over time, especially during high-demand periods. CONCLUSIONS: These results do not support the effect of performing a highly demanding cognitive task on subsequent strenuous physical performance. Instead, they reveal the richness of the subjective experience linked to cognitive performance that goes beyond mere mental fatigue. Overall, we show a novel way to understand the interplay between cognitive and physical performance.