Sample code using WordPress script_loader_tag filter to defer scripts

script_loader_tag.php

<?php
  // Do not load CF7 assets on other pages
  if ( class_exists('WPCF7') ) {
    add_filter( 'wpcf7_load_css', '__return_false' );
    //add_filter( 'wpcf7_load_js', '__return_false' );
    add_filter( 'wpcf7_use_really_simple_captcha', '__return_true' );

    // Add defer attribute
    add_filter( 'script_loader_tag', function ( $tag, $handle ) {
      if ( 'contact-form-7' !== $handle )
        return $tag;
        return str_replace( ' src', ' defer="defer" src', $tag );
    }, 10, 2 );
  }
?>