Lazyloading

You can use any library but on most projects we like using "lazysizes", suggested by Google on this page:

GitHub - aFarkas/lazysizes: High performance and SEO friendly lazy loader for images (responsive and normal), iframes and more, that detects any visibility changes triggered through user interaction, CSS or JavaScript without configuration.GitHub

Ideally (if there is more time) please check "the modern transparent srcset pattern" part of the documentation of "lazysizes".