lazysizes is a WordPress plugin for the high performance, SEO-friendly and auto-triggering lazyloader with the same name. Support includes images (including responsive images with srcset and the picture tag), iframes, scripts/widgets and much more. It also prioritizes resources by differentiating between crucial in view and near view elements to make perceived performance even faster. Additionally, you can add low-res/blurry placeholder images using the Blurhash algorithm.
This plugin works by loading the lazysizes script and replacing the src and srcset attributes with data-src and data-srcset on the front end of a WordPress site. When a post or page is loaded, the lazysizes javascript will load the images, iframes etc. dynamically when needed. All you need to do is to enable the plugin, and possibly tweak a few settings to your liking.
Thanks to aFarkas and contributors for making the lazysizes library possible, and for letting me use the same name.
Also thanks to dbhynds who made the Lazy Load XT plugin, which this plugin is based on.