There are three kinds of lazy-loading attributes but only two that really matter:
Lazy, which will be enabled in WordPress 5.4 by default, means to wait until the user’s browser viewport is within a certain distance before downloading the image. This behavior will speed up users’ experience on WordPress sites.
The “eager” attribute tells the browser to immediately download the image.
WordPress 5.4 Release Date
WordPress 5.4 is currently set to release on March 31, 2020. That date is subject to change depending on how ready the release is. But WordPress has been very good about meeting their deadlines.
According to the WordPress comments, the lazy-loading attributes will be implemented in a manner that makes it easy to remove them. This is important in case future browsers begin lazy-loading all images by default, without the need for a loading attribute.
Google has indicated via Web.dev that there are plans to automatically enable lazy-loading to all images when a browser in the Lite mode (formerly known as Data Saver mode).
Test Drive WordPress Lazy-Loading
The WordPress team has officially released a WordPress lazy-loading plugin designed for testing purposes. Publishers who want to test it and give feedback are welcome to download the plugin and give it a try.
Read the official WordPress announcement here: