lazySizes is the ultimate and lightweight lazyLoader which lazy loads images (including responsive images (picture/srcset)), iframes and scripts. It is written in VanillaJS and with high performance in mind.
LazySizes supports low quality image placeholder pattern (recommend for above the fold/crictical images), normal image pattern (for non-critical images or in case there is no low quality image available), responsive images with srcset and sizes attribute or responsive images with the picture element!
LazySizes can be extended to load nearly everything lazily, like iframes, videos and widgets/Javascript/script. Simply add the JS to your website and put the class lazyload to all elements, which should be lazy loaded.