The webhook of payment completion is triggered by the WP Cron. But the WP Cron is not a real-time cron worker. To solve the delay of order notification, you have 2 options. Option 1: Add the code add_filters( 'woocommerce_webhook_deliver_async', '__return_false' ); to the functions.php in the child theme of your store. Option 2: Install Openai Tools for WordPress & WooCommerce Plugin and check Webhook Delivery Async to enable the instant order notification.