This idea has been merged into another idea. To comment or vote on this idea, please visit WEBFLOW-I-318 Add option to 'lazy-load' images to improve load time, UX (and SEO).
I think this should be as simple as a tick-box for DIVs and other elements on the page so that anything beyond the viewport can be lazy loaded when the user gets into view. Obvious benefits out there for CMS as well but that shouldn't be a requirement. Even a simple site without CMS which includes heavy image/video content would be ideal for this feature.