AskTaimoor

All this plugin does is that it adds ‘async’ in all script tags it find on the page. 
This helps in loading the webpage much faster.

Async Javascript Optimizer

Adds async to all javascript links for fast loading of webpage.

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.

Async Javascript Optimizer vs Speed Up – Optimize CSS Delivery