Often when I'm watching a YouTube video the speaker is talking much slower than I can listen. For some videos, if you have joined the html5 beta, there is an option available under the settings menu to change playback speed to .5x, .75x, 1.5x or 2x but most videos lack this option. Here is a way to change the playback speed of any video to whatever you like.
First you have to go here and opt into the YouTube HTML5 beta.
Then, while watching the video, press ctrl + shift + j (command + option + j on a Mac), this will open the developer tools console which should look something like what you see below.
|The Developer Tools Console|
Paste the following text into the console and hit enter: document.getElementsByTagName("video").playbackRate = 1.4
This will change the playback speed to 1.4x.
To achieve a playback speed other than 1.4x just change the last number in the string to 1.6x or .25x or whatever you like.
Note: If there are loading errors on the page you may get a bar that pops up at the bottom of the screen instead of the developer tools console. Closing the bar and trying it again should fix this problem.