Psychological science remains unclear about how individuals' trait perfectionism impacts their performance—is more perfectionism linearly better or does too perfect backfire? The present study investigates a potentially non-linear relationship and its underlying mediators. Based on the two-dimensional model of perfectionism that distinguishes perfectionistic concerns (PC) versus strivings (PS), we investigate the (non-)linear relationships of perfectionism and performance in the letter detection task. Additionally, we experimentally examined whether time pressure would moderate these findings. Our study results (N = 229) establish non-linearity: a quadratic function in the form of an inverted U-shape best explains the relationship between perfectionistic concerns and performance. Contrary to our hypothesis, perfectionistic strivings predicted task performance linearly but negatively. Upon further examination, we also found empirical support for a combinatory effect of both dimensions: Only individuals high in PC showed the negative effect of PS on task performance. Although performance differed in the timed versus untimed task, time pressure did not moderate the (non-)linear relationships of PS or PC on performance. Multiple mediation analyses revealed that perceived distress, rumination, and effort mediated the quadratic relationships of perfectionistic concerns. Overall, our results question the strict disentanglement of perfectionistic dimensions and emphasise the usefulness of a more holistic approach.