The superiority of learner-paced over system-paced instructions was demonstrated in multiple experiments. In these experiments, the system-paced presentations were highly speeded, causing cognitive overload, while the learner-paced instructions allowed adjustments of the presentational flow to the learner's needs by pacing facilities, mostly leading to higher learning success. A research gap exists in investigating how effects depend on the speed of system-paced presentations. Hence, in this follow-up experiment to Stiller, Freitag, Zinnbauer and Freitag (2009), 142 university students received four web-based instructions consisting of static pictures and on-screen texts about the structure of the human eye. Three instructions were system-paced with a low, medium, and high speed, and one condition was learner-paced. Students rated their mental effort while learning and performed tests of retention, labelling pictures, verbalising structural knowledge, and transfer. With learner pacing, students reported lower mental effort and were better in labelling pictures and verbalising structural knowledge. Surprisingly, the medium system-paced instruction resulted in the worst performance, which partly questions common explanations of pacing effects. (Contains 2 figures and 5 tables.)