Published on 2025-06-28T02:13:07Z

What Is Asynchronous Loading? Examples for Analytics Implementations

Asynchronous Loading refers to the technique of fetching and executing scripts independently of the browser’s main HTML parsing process. In web analytics, this ensures that analytics scripts—such as Google Analytics 4 (GA4) or PlainSignal—do not block page rendering, leading to faster load times and smoother user experiences. By using HTML attributes like async and defer, alongside resource hints such as preconnect, developers can optimize when and how third-party code is loaded. This optimization is crucial not only for performance metrics like Largest Contentful Paint (LCP) and First Input Delay (FID) but also for maintaining data accuracy under various network conditions. Properly implemented asynchronous loading balances the need for real-time analytics with the imperative of web performance, ensuring that tracking remains reliable without compromising user experience.

Illustration of Asynchronous loading
Illustration of Asynchronous loading

Asynchronous loading

Load analytics scripts without blocking page rendering, boosting performance and ensuring accurate data collection.

Why Asynchronous Loading Matters

Asynchronous Loading prevents third-party scripts from delaying HTML parsing, which:

  • Enhances user experience by reducing initial load time and visual jank.
  • Improves Core Web Vitals such as LCP and FID.
  • Ensures analytics data is captured without impeding page interactivity.

Understanding these benefits is key for analytics engineers aiming to deliver both performance and accurate insights.

  • Non-blocking execution

    Async scripts are fetched in parallel without pausing HTML parsing, so page structure and styles load first.

  • Optimized resource loading

    Using preconnect and prefetch hints readies connections ahead of script requests, minimizing network latency.

Key Mechanisms of Asynchronous Loading

Three primary browser features enable asynchronous script loading:

  1. The async attribute for immediate, parallel fetch and execution.
  2. The defer attribute for parallel fetch and delayed execution post-parsing.
  3. Resource hints like preconnect to establish early connections.

  • The async attribute

    Marks a script for parallel download and execution as soon as it’s ready, regardless of HTML parsing state.

    • Behavior:

      <script async src="..."></script> executes immediately after download, potentially before document parsing completes.

  • The defer attribute

    Fetches the script in parallel but defers execution until after the entire HTML document is parsed.

    • Behavior:

      <script defer src="..."></script> guarantees script execution order and fires after DOMContentLoaded.

  • Preconnect hints

    Allows the browser to set up early connections (DNS, TLS, TCP) to external domains before resource fetch.

    • Implementation:

      Add <link rel="preconnect" href="//example.com" crossorigin /> in <head> to speed up subsequent requests.

Asynchronous Loading in Analytics Tracking

Implementing analytics with asynchronous loading ensures that data collection scripts do not compromise page performance. Below are examples with GA4 and PlainSignal:

  • Google analytics 4 (ga4) example

    GA4 uses the async attribute for non-blocking execution. Insert this in your <head>:

    • Tracking code: 
      <script async src="https://www.googletagmanager.com/gtag/js?id=GA_MEASUREMENT_ID"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());
  gtag('config', 'GA_MEASUREMENT_ID');
</script>

  • Plainsignal example

    PlainSignal provides a lightweight, cookie-free analytics snippet that uses defer and preconnect:

    • Tracking code: 
      <link rel="preconnect" href="//eu.plainsignal.com/" crossorigin />
<script defer data-do="yourwebsitedomain.com" data-id="0GQV1xmtzQQ" data-api="//eu.plainsignal.com" src="//cdn.plainsignal.com/PlainSignal-min.js"></script>

Best Practices and Considerations

To maximize the benefits of asynchronous loading, keep these guidelines in mind:

  • Fallbacks and error handling

    Provide fallback mechanisms in case scripts fail to load (e.g., timeouts, no-op functions), ensuring site stability.

  • Performance monitoring

    Continuously track performance metrics after implementing async/defer to detect regressions and verify timing improvements.

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