Hastebin

// ==UserScript==
// @name         YouTube Default Playback Speed
// @namespace    tim.local
// @version      1.2
// @description  Force a default playback speed via the YouTube player API
// @match        https://www.youtube.com/*
// @run-at       document-idle
// @grant        none
// ==/UserScript==

(function () {
    'use strict';

    const TARGET_RATE = 1.0; // long-term: 1.0

    function getPlayer() {
        const p = document.getElementById('movie_player');
        if (p && typeof p.setPlaybackRate === 'function' && typeof p.getPlaybackRate === 'function') {
            return p;
        }
        return null;
    }

    function enforce() {
        const start = Date.now();
        const iv = setInterval(() => {
            const p = getPlayer();
            if (p && p.getPlaybackRate() !== TARGET_RATE) {
                p.setPlaybackRate(TARGET_RATE);
            }
            if (Date.now() - start > 6000) clearInterval(iv);
        }, 250);
    }

    enforce();
    document.addEventListener('yt-navigate-finish', enforce);
})();