如何延迟加载滑动滑块的每个幻灯片？

Question

以下巧妙的滑块代码可对图像进行延迟加载，但在Google PageSpeed Insights上仍然会延迟屏幕外图像问题。 有谁知道如何解决滑动页面滑块在Google页面速​​度上的“延迟屏幕外图像”问题？

<div class="demo">
   <div>
       <a href="#" class="block-promo home-main">
          <img src="" data-lazy="https://via.placeholder.com/150"/>
       </a>
   </div>
   <div>
      <a href="#" class="block-promo home-main">
         <img src="" data-lazy="https://via.placeholder.com/150"/>
      </a>
   </div>
   <div>
      <a href="#" class="block-promo home-main">
         <img src="" data-lazy="https://via.placeholder.com/150"/>
      </a>
   </div>
</div>


<script>

      $('.demo').slick({
       slidesToShow: 1,
       slidesToScroll: 1,
       dots:false,
       lazyLoad: 'ondemand',
       autoplay: true,
       autoplaySpeed: 5000,
       initialSlide: -1,
       infinite: true,
       arrows: true
       });   

</script>