function async_user_register( $user_id ) {

    $args = array(
        'class'    => 'WPF_User',
        'function' => 'user_register',
        'args'     => array( $user_id, null )
    );

    wp_fusion()->ajax->async_add( $user_id, $args );

}

add_action( 'user_register', 'async_user_register' );

remove_action( 'user_register', array( wp_fusion()->user, 'user_register' ), 20 );