selectively-defer-js.php

<?php

/**
 * The only point of this is to add the `defer` attribute to
 * a script (in this case, jQuery) to test with.
 */
add_action( 'wp_print_scripts', function() {
	wp_script_add_data( 'jquery-core', 'defer', true );
} );

/**
 * This is the meat and potatoes -- checks for defer attribute and adds it to the tag.
 */
add_filter( 'script_loader_tag', function( $tag, $handle, $src ) {
	$scripts = wp_scripts();
	$obj = $scripts->registered[ $handle ];

	if ( ! empty( $obj->extra['defer'] ) ) {
		$tag = str_replace( " src='$src'>", " src='$src' defer>", $tag );
	}

	return $tag;
}, 10, 3 );