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  …

Chirag Swadia

WordPress Lazy Load plugin enhances your page load speed by not loading the images until they are visible to the user.
This works for all your images, whether they are in post content, sidebar or even any custom image you might have added in your blog.
There are no settings or code changes required on your end. Just activate the plugin and you are good to go!

WordPress Lazy Load

WordPress Lazy Load plugin enhances your page load speed by not loading the images until they are visible to the user

Featured Image Sharpen Up vs WordPress Lazy Load