Plugin link: https://wordpress.org/plugins/lazy-op...
In this video we will see how to lazy load or defer background images that are in the external CSS files using a plugin called "lazy optimization" plugin. "lazy optimization" plugin works on top of "Autotimize" plugin. "lazy optimization" plugin will help in the lazy loading or deferring of background images that are in the external CSS files generated by "Autoptimize" plugin.
By lazy loading background images your website's pages overall size will decrease and will help to reduce/optimize your website loading time.