mabujo

Featured Image Sharpen Up makes a tiny version of your featured post image and places it in the head of the page. 
As you scroll down the page, some JavaScript lazy loads the featured images in the background and replaces the tiny images as the full images are loaded.
You may have seen a similar effect used for images on medium.com.
Created on WordPress 4.4, but should work on earlier versions. 
Tested working on all Twenty Sixteen, Twenty Fifteen, Twenty Fourteen, Twenty Thirteen, Twenty Twelve and Twenty Eleven themes. Should work on other themes, but may need CSS adjustments, depending on theme handling of featured images.
Uses jQuery.Lazy(); (http://jquery.eisbehr.de/lazy/) and Hugh Lashbrooke’s plugin template (https://github.com/hlashbrooke/WordPress-Plugin-Template)
Plugin page : <a href="https://mabujo.com/blog/featured-image-sharpen-up-wordpress-plugin/" rel="nofollow ugc">Featured Image Sharpen Up plugin @ mabujo</a>

Featured Image Sharpen Up

This plugin helps increase the page loading speed of your site by replacing featured post images with a small inline image, and lazy loading the full  …

Oxyplug Team

<ul>
<li>Supported only on Chromium-based browsers since version 121+</li>
<li>In the post/product category pages, first X items will be prefetched before the user clicks. The default value for X is 4 items and can be adjusted via the plugin settings.</li>
<li>If a user hovers over a link, the destination page will be prefetched before click. Please note that this option is disabled by default and can be enabled via the plugin settings.</li>
<li>It is also possible to prefetch some custom links on posts/products by entering preferred URLs.</li>
<li>Similar actions can be applied with prerender.</li>
</ul>

Oxyplug Prefetch & Prerender

Faster loading next pages by prerendering/prefetching all links a user hovers or addresses you prefer. It improves UX and Core Web Vitals score.

Featured Image Sharpen Up vs Oxyplug Prefetch & Prerender