Video Speed Optimization - Replace Youtube Video with Thumbnail, Load Video on Thumbnail Click

youtube-replace-video-embed-thumbnail.css

.youtube-player {
    position: relative;
    padding-bottom: 56.23%;
    /* Use 75% for 4:3 videos */
    height: 0;
    overflow: hidden;
    max-width: 100%;
    background: #000;
}

.youtube-player iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 100;
    background: transparent;
}

.youtube-player img {
    bottom: 0;
    display: block;
    left: 0;
    margin: auto;
    max-width: 100%;
    width: 100%;
    position: absolute;
    right: 0;
    top: 0;
    border: none;
    height: auto;
    cursor: pointer;
    -webkit-transition: .4s all;
    -moz-transition: .4s all;
    transition: .4s all;
}

.youtube-player img:hover {
    -webkit-filter: brightness(75%);
}

.youtube-player .play {
    height: 72px;
    width: 72px;
    left: 50%;
    top: 50%;
    margin-left: -36px;
    margin-top: -36px;
    position: absolute;
    background: url('//i.imgur.com/TxzC70f.png') no-repeat;
    cursor: pointer;
}

youtube-replace-video-embed-thumbnail.html

<div class="youtube-player" data-id="VIDEOID"></div>

youtube-replace-video-embed-thumbnail.js

<script>

    /* Light YouTube Embeds by @labnol */
    /* Web: http://labnol.org/?p=27941 */

    document.addEventListener("DOMContentLoaded",
        function() {
            var div, n,
                v = document.getElementsByClassName("youtube-player");
            for (n = 0; n < v.length; n++) {
                div = document.createElement("div");
                div.setAttribute("data-id", v[n].dataset.id);
                div.innerHTML = labnolThumb(v[n].dataset.id);
                div.onclick = labnolIframe;
                v[n].appendChild(div);
            }
        });

    function labnolThumb(id) {
        var thumb = '<img src="https://i.ytimg.com/vi/ID/maxresdefault.jpg">',
            play = '<div class="play"></div>';
        return thumb.replace("ID", id) + play;
    }

    function labnolIframe() {
        var iframe = document.createElement("iframe");
        var embed = "https://www.youtube.com/embed/ID?modestbranding=1&color=red&autoplay=1&controls=1&rel=0&wmode=transparent";
        iframe.setAttribute("src", embed.replace("ID", this.dataset.id));
        iframe.setAttribute("frameborder", "0");
        iframe.setAttribute("allowfullscreen", "1");

        iframe.setAttribute("allow", "accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture");

        this.parentNode.replaceChild(iframe, this);
    }

</script>