TL;DR: faster playback can translate to better engagement, retention, and much quicker progress.

Average adult reads prose text at 250 to 300 words per minute (wpm). By contrast, the average rate of speech for English speakers is ~150 wpm, with slide presentations often closer to 100 wpm. As a result, when given the choice, many viewers speed up video playback to 1.31.5 its recorded rate to compensate for the difference.

Better, many viewers report that accelerated viewing keeps their attention longer: faster delivery keeps the viewer more engaged with the content. With a little training, many end up watching videos at speeds twice the normal rate and even higher. In fact, some studies report that after being exposed to accelerated playback, listeners become uncomfortable if they are forced to return to normal rate of presentation.

HTML5 video provides a native API to accelerate playback of any video. The problem is, many players either hide, or limit this functionality. For best results playback speed adjustments should be easy and frequent to match the pace and content being covered: we don't read at a fixed speed, and similarly, we need an easy way to accelerate the video, slow it down, and quickly rewind the last point to listen to it a few more times.

Once the extension is installed simply navigate to any page that offers HTML5 video (example), and you'll see a speed indicator in top left corner. Hover over the indicator to reveal the controls to accelerate, slowdown, or rewind the video (10 seconds + lowers playback speed). Or, even better, simply use your keyboard:

• a - will rewind video 10s and lower playback speed.
• s - will lower playback speed.
• d - will accelerate playback speed.

Also, a few tips for enabling / forcing HTML5 video:

• YouTube: make sure you enable the HTML5 opt-in experiment.
• Udemy: account settings > preferences > HTML5 player (instructions).
• If you're adventerous, try disabling the Flash plugin in Chrome: chrome://plugins/

• Make it not suck with Vimeo content: "s" shortcut and button clicks
• Some YouTube videos still fallback to flash: force iOS user-agent to force HTML5?
• Add plumbing for configurable shortcuts?
• Your awesome feature here...

(MIT License) - Copyright (c) 2014 Ilya Grigorik