Lazy Load

add_action('wp_head', 'custom_lazyload_script');

function custom_lazyload_script(){

  ?>

  <script>

    window.onload = function() {

      document.querySelectorAll("img[data-src]").forEach(function(image) {

        image.setAttribute('src', image.getAttribute('data-src'));

        image.onload = function() {image.removeAttribute('data-src');};

      });

    };

  </script>

  <?php

}