Speed up videos on the web

If the website uses the <video> tag from HTML5, simply open a console in the browser and run one of the following commands:

document.getElementsByTagName("video")[0].playbackRate = 2.5;

document.querySelectorAll("video")[0].playbackRate = 3.0;

Although sites like YouTube and Netflix already have buttons to change the playback rate to predefined values, with this command, you can use any speed.

In Firefox, the command to open a console in the Web Developer Tools is Ctrl+Shift+K.

There are also browser add-ons that do the same thing.
E.g.: Video Speed Controller for Firefox.