I always forget how to do this, so I’m going to write it down so I can look it up later. When a video is playing, right-click the volume control slider and select Enhancements, then Play speed settings. (Actually, it also works if you right-click the speaker icon, the Stop button, the Replay button, or the Shuffle button, but the volume control slider is the biggest target.) On the Play speed settings dialog, the single-step controls are at the bottom; they look like triangles.

Update: There’s an even easier way.