Derrick Hammer

This plugin will automatically have the CriticalCSS.com web service get the needed above the fold CSS for every page on your site to help improve your user experience and site speed. This is commonly required by google pagespeed as one of the last steps to do.
This plugin alone will not improve site performance. You need a minify and/or caching plugin as well.
This plugin does not handle any minification or re-ordering of assets. I recommend using <a href="https://wp-rocket.me/" rel="nofollow ugc">WP Rocket</a> with my <a href="https://wordpress.org/plugins/rocket-footer-js/">WP Rocket Footer JS</a>, <a href="https://wordpress.org/plugins/rocket-async-css/">WP Rocket ASYNC CSS</a>, and the plugin <a href="https://wordpress.org/plugins/the-preloader/">Preloader</a>
If you are looking for a professional team to get your WordPress site to run faster, check us out for our speed optimization services at <a href="https://rankgrowdigital.com/" rel="nofollow ugc">Rank Grow Digital</a>

WP Critical CSS

Triggers a fatal PHP error:

Fatal error: Uncaught ArgumentCountError: array_merge() does not accept unknown named parameters in public_html/wp-content/plugins/wp-criticalcss/lib/WP/CriticalCSS/Request.php:119

Cannot activate

WordPress plugin that will use CriticalCSS.com to automatically generate above-the-fold CSS for your pages.

bypr.nils

Smart Image Loader loads images which are visible in the initial viewport of your website before any images whose position is “below the fold”, outside the current viewport. Those images can be loaded as soon as the visible images are finished loading or “lazy loaded” when they would become visible. This can be useful for bandwidth saving on mobile devices.
There is no need to insert any additional code into your website, just install the plug-in.
The default settings are fine in most cases, but you may want to adjust them for optimization.
Smart Image Loader is tested and works down to Internet Explorer 7, disabled below.
Note: there is currently no support for (CSS) background images.

Smart Image Loader

Smart Image Loader is a fire-and-forget priority and lazy loader for image sources. Can be a huge performance boost especially for one pagers.

WP Critical CSS vs Smart Image Loader