Preload largest contentful paint image

In general, it's very important to have the most important resources of your website available from an early stage of the page load. However, some of these important assets might be loaded from a CSS or JavaScript file and this may take longer than desired.

When an image is the Largest Contentful Paint element, PageSpeed may recommend preloading it. 

In that case, Optimize critical images WP Rocket feature will automatically instruct the browser to preload the image, to load it with priority, and to prevent it from being lazyloaded.

Please see this article for more information.