In some cases difficult-to-read learning materials lead to better performance than easy to-read materials. This phenomenon is called the disfluency effect and is attributed to a subjective feeling of task difficulty resulting in a deeper learning approach. In two studies (n[subscript 1] = 67, n[subscript 2] = 73) we investigated the impact of three increasing disfluency levels compared to a fluent condition on learning outcomes, in the second study we additionally addressed effects on cognitive load and time on task. The first study revealed significant differences of the experimental groups and on a descriptive level a reversed u-shape pattern, i.e., learning performance increases with increasing disfluency but decreases when the text becomes too illegible. The same pattern could be found significantly in the second study for recall and transfer performance, engagement and time on task and the reversed pattern for extraneous load. Results indicate that there is in fact an optimal level of disfluency on performance that comes along with increased engagement and that there is a breaking point of disfluency where the extraneous load seems to be too high.