We explore the relationship between video playback speed and student learning outcomes. Using an experimental design, we present the results of a pre-registered study that assigns users to watch videos at either 1.0x or 1.25x speed. We find that students who consume sped content are more likely to get better grades in a course, attempt more content, and obtain more certificates. We also find that when videos are sped up, students spend less time consuming videos and are marginally more likely to complete more video content. These findings suggest that future study of playback speed as a tool for optimizing video content for MOOCs is warranted. Applications for reinforcement learning and adaptive content are discussed. [This paper was published in: "Proceedings of the 10th International Conference on Learning Analytics and Knowledge (LAK '20)," Association for Computing Machinery, March 23-27, 2020, pp. 260-269.]