http://shpargalkablog.ru/2013/06/youtube.html
https://www.cssscript.com/lazy-load-iframe-embedded-content-javascript-lazyframe-js/

<div
	class="lazyframe"
	data-src="https://www.youtube.com/embed/6hgVihWjK2c"
	data-thumbnail="//img.youtube.com/vi/6hgVihWjK2c/hqdefault.jpg"
	data-vendor="youtube"
    >
</div>

<div class="lazyframe lazyframe--loaded" data-src="https://www.youtube.com/embed/6hgVihWjK2c" data-vendor="youtube"></div>
<script>
 lazyframe('.lazyframe', {
      apikey: 'AIzaSyAYRnB2Iyqsi9z5HWtEVty3Sad5n9fsLZ0' // Sorry, will only work on this domain
    });
</script>