When the defer attribute is present in the script tag, it specifies that the script is executed when the page has finished parsing. A requirement that is enforced by Google for example (Page-Speed ranking).

Currently the WordPress wp_enqueue_script provides no easy way to add new attributes, but there is a way around that :)
The below hack / filter needs to be added to your theme function.php. The filter should be your preferred solution, as the clean_url filter has been deprecated.

Make sure that your website / theme is still loading after adding these changes. If needed you can add more exceptions, as I did for jQuery.

Hack for WordPress before 4.1

if(is_admin() === FALSE){
function defer_parsing_of_js ( $url ) {
if ( FALSE === strpos( $url, '.js' ) ) return $url;
if ( strpos( $url, 'jquery.js' )  ) return $url;
return "$url' defer ";
}
add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 );
}

if(is_admin() === FALSE){

function defer_parsing_of_js ( $url ) {

if ( FALSE === strpos( $url, '.js' ) ) return $url;

if ( strpos( $url, 'jquery.js' ) ) return $url;

return "$url' defer ";

}

add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 );

}

Filter for WordPress 4.1+
Updated: 26.04.2015 – using clean Regexp

if(is_admin() === FALSE){
function add_js_defer($tag, $handle, $src){
 if(  strpos( $tag, 'jquery.js' ) ) return $tag;
  preg_match_all('/([\w\-.:]+)\s*=\s*("[^"]*"|\'[^\']*\'|[\w\-.:]+)/i', $tag, $match);
		
	$newtag = "<script ";
	
	foreach($match[1] as $key=> $val){
		$newtag .= $val."=".$match[2][$key]." ";	
	}
	
	$newtag .= "defer></script>";
	
    return $newtag.PHP_EOL;
 }
 add_filter( 'script_loader_tag', 'add_js_defer', 11, 1 );
}

if(is_admin() === FALSE){

function add_js_defer($tag, $handle, $src){

if( strpos( $tag, 'jquery.js' ) ) return $tag;

preg_match_all('/([\w\-.:]+)\s*=\s*("[^"]*"|\'[^\']*\'|[\w\-.:]+)/i', $tag, $match);

$newtag = "<script ";

foreach($match[1] as $key=> $val){

$newtag .= $val."=".$match[2][$key]." ";

}

$newtag .= "defer></script>";

return $newtag.PHP_EOL;

}

add_filter( 'script_loader_tag', 'add_js_defer', 11, 1 );

}

Browser Support

All modern browsers
Support in IE9 is flaky, as the execution order isn’t guaranteed.
Can I use

Defer Parsing of JavaScript in WordPress – new filter & old hack

Browser Support

Alexander