Manually defer the parsing of JS files in WordPress. Depending on the theme and installed plugins, it may be better to let a plugin handle this.

functions.php

<?php
/**
 * Defer all JS files
 *
 * @method defer_parsing_of_js
 * @param  String              $url The URL of the script
 * @return String                   The "cleaned" URL, including the additional defer attribute
 */
function defer_parsing_of_js( $url ) {
	// If the URL is not of a .js file, just return it
	if ( false === strpos( $url, '.js' ) ) {
		return $url;
	}

	// If the URL is for jQuery, just return that also
	if ( strpos( $url, 'jquery.js' ) ) {
		return $url;
	}

	// Otherwise, return the URL plus the defer attribute. The onload is really just a hack to get the URL to close properly
	return "{$url}' defer onload='";
}

// Only defer the JS when viewing the front end of the site
if ( !is_admin() ) {
	add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 );
}