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ran / auto adjust youtube video speed 

// ==UserScript==
// @name auto adjust youtube video speed
// @namespace Violentmonkey Scripts
// @description set youtube video speed to 1.5x(you can set your own speed) automatically.
// @match *://*.youtube.com/*
// @require  http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js
// @require  https://gist.github.com/raw/2625891/waitForKeyElements.js
// @grant    GM_addStyle
// @version 0.0.1.20180625212538
// @copyright 2018, ran (https://openuserjs.org/users/ran)
// @license MIT
// ==/UserScript==

waitForKeyElements("video", function () {
  'use strict';

  // set your desired play speed here
  var rate = 1.5;
  document.querySelector('video').playbackRate = rate;
});