nigro.simone

This small plugin (5 Kb) loads the stylesheets asynchronously and improve page load times.
The recommended use of this plugin is to load your vital stylesheets synchronously and non-vital CSS files asynchronously. 
Non-vital CSS-files can be for example: fonts, icons, before the fold template-specific CSS, etc.
You can choose which files to load synchronously with a filter in your function.php, eg.:
<pre><code>// exclude main and child stylesheets from delivery optimization
function exclude_from_delivery_optimization($handle){
 return in_array($handle, array('main-stylesheet', 'child-stylesheet'));
}
add_filter('speed-up-optimize-css-delivery', 'exclude_from_delivery_optimization');
</code></pre>
Note: this only works if your other plugins and theme add the CSS correctly.

Speed Up – Optimize CSS Delivery

This plugin load the stylesheets asynchronously and improve page load times.

Rick Buczynski

Easily add scripts and stylesheets per post. This is a great tool for developers creating custom pages for their clients, but also useful for the blogger who wants to separate inline CSS & JS from their post content.
This plugin provides an interface for wp_enqueue_script and wp_enqueue_style functions, so all of the functionality that a developer would find there is available in this plugin.
However, script localization differs from wp_localize_script in that with this plugin you are allowed to input raw JavaScript code for localization, library initialization, and more.
This plugin may also help to alleviate modern browser XSS detection from blocking potentially unsafe JavaScript.

Post Head Includes

Easily add scripts and stylesheets to the HEAD of your posts, keeping your HTML cleaner without inline scripts or styles.

Speed Up – Optimize CSS Delivery vs Post Head Includes