Within WordPress 5.4, one of the latest features is called Lazy Loading. Lazy Loading means to wait until the user’s browser viewport is within a certain distance before downloading image/videos creating a faster website experience.
This is how Lazy Loading works:
The browser builds the web page Document Object Model without downloading images and preloading videos.
Downloading and rendering of additional videos are delayed until a site visitor scrolls down the page and additional content comes into view.
By using Lazy Loading, the images/videos are not loaded until needed. This can provide a significant boost in performance for sites that include a lot of high-resolution images and embedded videos.