How do I defer or async this WordPress javascript snippet to load lastly for faster page load times?

I have various javascripts that are necessary plugins in one of my WordPress domains, and I know where in the php file it's called from.

I'm taking every measure I can take to speed up page loading times, and every speed tester on the web says to defer javascripts if possible.

I have read about the defer='defer' and the async functions in javascript and I think one of these will accomplish what I'm trying to accomplish. But I'm not understanding how I'd do so in a php file.

For instance, here is a snippet from one particular plugin's php file where the javascript file is being called:

function add_dcsnt_scripts() {

    wp_enqueue_script( 'jquery' );
    wp_enqueue_script( 'dcsnt', dc_jqsocialtabs::get_plugin_directory() . '/js/jquery.social.media.tabs.1.7.1.min.js' );

}

I've read that it's best to do something like this for faster page loading times:

<script defer async src="..."></script>

But I don't know how to accomplish that within a php file. I want to do this with all of my javascript files.

How would I accomplish deferring or asyncing this javascript snippet to is loads last and speeds up page load times? What would be the ideal way to increase page load times across all browsers? Thanks for any guidance anybody can offer!

Or more universal way:

function add_async_forscript($url)
{
    if (strpos($url, '#asyncload')===false)
        return $url;
    else if (is_admin())
        return str_replace('#asyncload', '', $url);
    else
        return str_replace('#asyncload', '', $url)."' async='async"; 
}
add_filter('clean_url', 'add_async_forscript', 11, 1);

so you can add async to any script without code changes, just add #asyncload to script url as:

wp_enqueue_script('dcsnt', '/js/jquery.social.media.tabs.1.7.1.min.js#asyncload' )

Trying to keep things somewhat modular and all encompassing, the following approach dynamically chooses how to embed the tag with the async or defer attributes by just appending a small identifier to the $handle name:

/**
* Add async or defer attributes to script enqueues
* @author Mike Kormendy
* @param  String  $tag     The original enqueued <script src="...> tag
* @param  String  $handle  The registered unique name of the script
* @return String  $tag     The modified <script async|defer src="...> tag
*/
// only on the front-end
if(!is_admin()) {
    function add_asyncdefer_attribute($tag, $handle) {
        // if the unique handle/name of the registered script has 'async' in it
        if (strpos($handle, 'async') !== false) {
            // return the tag with the async attribute
            return str_replace( '<script ', '<script async ', $tag );
        }
        // if the unique handle/name of the registered script has 'defer' in it
        else if (strpos($handle, 'defer') !== false) {
            // return the tag with the defer attribute
            return str_replace( '<script ', '<script defer ', $tag );
        }
        // otherwise skip
        else {
            return $tag;
        }
    }
    add_filter('script_loader_tag', 'add_asyncdefer_attribute', 10, 2);
}

Example usage:

function enqueue_my_scripts() {

    // script to load asynchronously
    wp_register_script('firstscript-async', '//www.domain.com/somescript.js', '', 2, false);
    wp_enqueue_script('firstscript-async');

    // script to be deferred
    wp_register_script('secondscript-defer', '//www.domain.com/otherscript.js', '', 2, false);
    wp_enqueue_script('secondscript-defer');


    // standard script embed
    wp_register_script('thirdscript', '//www.domain.com/anotherscript.js', '', 2, false);
    wp_enqueue_script('thirdscript');
}
add_action('wp_enqueue_scripts', 'enqueue_my_scripts', 9999);

Outputs:

<script async type='text/javascript' src='//www.domain.com/somescript.js'></script>
<script defer type='text/javascript' src='//www.domain.com/otherscript.js'></script>
<script type='text/javascript' src='//www.domain.com/anothercript.js'></script>

Thanks to @MattKeys @crissoca for inspiring my answer here.

This blog post links to two plugins of interest:

Asynchronous Javascript
Improve page load performance by asynchronously loading javascript using head.js

WP Deferred Javascripts
Defer the loading of all javascripts added with wp_enqueue_scripts, using LABJS (an asynchronous javascript library).

Haven't tested them but checked the code and they do pretty fancy stuff with WordPress scripts enqueuing process.

But then WPSE comes to rescue:

// Adapted from https://gist.github.com/toscho/1584783
add_filter( 'clean_url', function( $url ) {
    if ( FALSE === strpos( $url, '.js' ) ) {
        // not our file
        return $url;
    }
    // Must be a ', not "!
    return "$url' defer='defer";
}, 11, 1 );

Another solution using a different filter, which can be used to target a specific script handle:

function frontend_scripts()
{
    wp_enqueue_script( 'my-unique-script-handle', 'path/to/my/script.js' );
}
add_action( 'wp_enqueue_scripts', 'frontend_script' );

function make_script_async( $tag, $handle, $src )
{
    if ( 'my-unique-script-handle' != $handle ) {
        return $tag;
    }

    return str_replace( '<script', '<script async', $tag );
}
add_filter( 'script_loader_tag', 'make_script_async', 10, 3 );

A simplified method. Add to your functions.php file to make make JavaScript asynchronous in Wordpress

// Make JavaScript Asynchronous in Wordpress
add_filter( 'script_loader_tag', function ( $tag, $handle ) {    
    if( is_admin() ) {
        return $tag;
    }
    return str_replace( ' src', ' async src', $tag );
}, 10, 2 );