Using a caching system, Contact Form 7 sends an AJAX request for each form (taking at least 500ms) in order to handle Really Simple Captcha and Quiz Captcha updates. The request is being sent even using Google Recaptcha, which is useless. We're going to disable this unwanted behaviour.

functions.php

/**
 * Disables REFILL function in WPCF7 if Recaptcha is in use
 */

add_action('wp_enqueue_scripts', 'wpcf7_recaptcha_no_refill', 15, 0);
function wpcf7_recaptcha_no_refill() {
  $service = WPCF7_RECAPTCHA::get_instance();
	if ( ! $service->is_active() ) {
		return;
	}
  wp_add_inline_script('contact-form-7', 'wpcf7.cached = 0;', 'before' );
}