Free WordPress Lazy Load by WP Rocket pluginFree
Lazy Load by WP Rocket displays images and/or iframes on a page only when they are visible to the user. This reduces the number of HTTP requests mechanism and improves the loading time.
You can also replace Youtube iframes by a preview thumbnail to further speed up the loading time of your website.
Lazyload script: https://github.com/verlok/lazyload
Author: WP Media
Source from: WordPress.org