<?php 
  add_filter( 'the_content', 'lazy_iframes' );
  function lazy_iframes($content) {
      if ( is_single() ) {
          $content = preg_replace('/<iframe src="(.*)"/i', '<iframe data-iframe-src="$1"' , $content);
      }
      return $content;
  }
  
 JS
 const frames = document.querySelectorAll("[data-iframe-src]");
  if (frames.length) {
    setTimeout(
      () => frames.forEach((frame) => (frame.src = frame.dataset.iframeSrc)),
      1300
    );
  }

