Uses jQuery.sonar to only load an image when it's visible in the viewport.
https://wordpress.org/plugins/lazy-load/
Widgets · WordPress Plugins
Get access to data on 218,623 websites that are Lazy Load for WordPress Customers. We know of 73,143 live websites using Lazy Load for WordPress and an additional 145,480 sites that used Lazy Load for WordPress historically and 40,231 websites in the United States.
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Get a list of 218,623 websites using Lazy Load for WordPress which includes location information, hosting data, contact details, 73,143 currently live websites and an additional 33,558 domains that redirect to sites in this list. 145,480 sites that used this technology previously and 40,231 websites in the United States currently using Lazy Load for WordPress.