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  …

Rex Anthony Eubanas (Batugan Design)

Fast and optimized pages lead to higher visitor engagement, retention, and conversions. The Page Speed Plugin help you to get a passing grade in Google Page Speed Insight.
For More Info Visit <a href="https://developers.google.com/speed/pagespeed/insights/" rel="nofollow ugc"> Google PageSpeed Insights </a>
Features include:
<ul>
<li>Enable compression</li>
<li>Leverage browser caching</li>
<li>Eliminate render-blocking resources</li>
<li>Remove unused JavaScript</li>
<li>Remove unused CSS</li>
<li>Serve images in next-gen formats</li>
<li>Properly Size Images</li>
<li>Defer Offscreen Images</li>
<li>
Efficiently encode images
</li>
<li>
Reduce the impact of third-party code
</li>
</ul>
Future Development:
<ul>
<li>Reduce server response times (TTFB)</li>
<li>Ensure text remains visible during webfont load</li>
<li>AMP Ready</li>
</ul>

Page Speed

useless

Fast and optimized pages lead to higher visitor engagement, retention, and conversions.

Featured Image Sharpen Up vs Page Speed