How Asynchronous Loading Improves Page Speed and Core Web Vitals
As marketers, we often face the challenge of building fast, responsive websites while integrating third-party elements like social sharing buttons, analytics tools, or real-time data. These tools can significantly impact performance—especially if loaded synchronously.
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Synchronous vs. Asynchronous
Understanding the difference between synchronous and asynchronous loading is essential if you want to optimize for both speed and user experience. The way you handle JavaScript loading—particularly third-party scripts—can directly affect Google’s Core Web Vitals (CWV), which measure real-world performance like load time, interactivity, and layout stability.
The visual below demonstrates the difference:
Synchronous
Synchronous scripts block page rendering. When a script is included without any modifiers, the browser must stop parsing the page, download the script, and execute it before continuing. This behavior can cause significant delays—especially when loading third-party content from social networks, ad servers, or analytics platforms.
Asynchronous
Asynchronous scripts, on the other hand, allow the browser to continue parsing HTML while the script is downloaded and executed in the background. This reduces the impact on render time and helps critical content load sooner.
There are two main approaches to loading scripts asynchronously:
- Using browser-supported
asyncordeferattributes - Injecting scripts after the page has finished loading (post-load execution)
Both approaches help avoid blocking the main content from rendering.
Why This Matters for Core Web Vitals
Google’s Core Web Vitals focus on three performance areas:
- Largest Contentful Paint (LCP): How quickly the main content becomes visible
- First Input Delay (FID): How soon the page responds to user interaction
- Cumulative Layout Shift (CLS): How stable the layout is while loading
Synchronous scripts hurt all three. They delay the loading of visible content, block interactions, and often introduce layout shifts when late-loading elements like social buttons or ads are injected above the fold.
Asynchronous techniques allow non-critical scripts to load without interfering with the primary rendering process, helping your CWV scores and, in turn, your search performance.
Native async and defer Attributes
Modern browsers support two key attributes that allow you to load JavaScript without blocking the rest of the page: async and defer. Both attributes are added to a <script> tag to change how and when the script is downloaded and executed—but they behave differently, and it’s essential to choose the right one based on your use case.
What Happens Without Either Attribute?
When you include a script like this:
<script src="script.js"></script>The browser stops parsing the HTML, fetches the script, executes it immediately, and then resumes rendering the rest of the page. This blocks the page load, delaying content rendering and user interaction—especially bad for performance and Core Web Vitals.
The async Attribute
<script async src="https://example.com/script.js"></script>With async, the browser downloads the script in parallel with parsing the HTML. However, as soon as the script finishes downloading, it pauses HTML parsing to execute the script immediately.
Key traits of async:
- Scripts load independently and run as soon as they’re ready
- Execution order is not guaranteed—whichever script loads first runs first
- Best used for standalone scripts that don’t rely on other scripts or DOM elements
Good examples: Analytics scripts, ad network tags, tracking pixels
The defer Attribute
<script defer src="https://example.com/script.js"></script>With defer, the browser downloads the script in parallel just like async, but defers its execution until after the HTML has been fully parsed (just before the DOMContentLoaded event fires).
Key traits of defer:
- Scripts load in parallel with HTML parsing
- Scripts execute in the order they appear in the HTML
- Ideal for scripts that interact with DOM elements or depend on each other
Good examples: Your site’s core JavaScript files, libraries like jquery.js followed by dependent scripts
Quick Comparison
| Attribute | Downloads in Parallel? | Execution Order | Blocks Rendering? | Executes When? |
|---|---|---|---|---|
| None | No | As seen | Yes | Immediately |
async | Yes | Unpredictable | No | As soon as ready |
defer | Yes | As seen in HTML | No | After HTML parsed |
By understanding and using these attributes correctly, you can significantly improve your site’s perceived speed. Always ask: Does this script need to run immediately and in order? Or can it wait until the page is visible to the user?
Whenever possible, use defer for your scripts and reserve async for isolated, non-blocking third-party tools. If a script doesn’t offer either option, consider loading it manually using JavaScript or via Google Tag Manager after the page has fully loaded.
Post-Load Script Injection with JavaScript
To fully control when scripts load—and prevent any negative impact on performance—you can inject them after the page is fully loaded.
(function() {
function async_load(){
var s = document.createElement('script');
s.type = 'text/javascript';
s.async = true;
s.src = 'https://example.com/social-buttons.js'; // Replace with actual script URL
var x = document.getElementsByTagName('script')[0];
x.parentNode.insertBefore(s, x);
}
if (window.attachEvent) {
window.attachEvent('onload', async_load);
} else {
window.addEventListener('load', async_load, false);
}
})();This pattern ensures the script doesn’t run until everything else is fully loaded. Even if the third-party script is synchronous by nature, loading it this way prevents it from blocking your page.
Use case: Load social sharing buttons, dynamic pricing scripts, or tracking pixels that don’t need to be visible immediately.
jQuery Example for Post-Load Injection
If your site uses jQuery, the same logic becomes simpler:
$(window).on('load', function() {
$.getScript("https://example.com/social-buttons.js");
});Or dynamically inject HTML:
$(window).on('load', function() {
$("#placeholder").load("/ajax/live-count.php");
});Use case: After the page loads, insert a live statistic or user count into a specific element.
Google Tag Manager: Load After Page Load
Google Tag Manager (GTM) lets you configure tags to fire after the page is fully loaded, helping ensure they don’t interfere with your LCP or CLS.
Loaded Trigger
- Create a Custom HTML Tag: Paste your script or iframe:
<script async src="https://example.com/social-buttons.js"></script> - Create a Trigger: Choose Window Loaded as the trigger event. This ensures the script doesn’t run until all assets are loaded.
- Publish Your Changes
Use case: Load tracking pixels, dynamic content tools, or even A/B testing scripts that aren’t needed for the initial render.
Putting It Into Practice
On Martech Zone, we’ve used this exact approach to defer all non-essential third-party scripts. Previously, if the script’s source lagged, the entire page load would suffer. Now, with asynchronous loading and post-load execution, the core content renders quickly, and social features load afterward without affecting performance or stability.
We’ve seen Core Web Vitals improve and user engagement rise, as visitors can interact with the page much sooner.
Conclusion
If your website loads scripts synchronously—especially third-party ones—you’re likely hurting your performance and your rankings. Modern web performance is about prioritizing the user experience: render first, enhance second.
Asynchronous loading and post-load script injection are powerful techniques to balance functionality with speed. Whether you’re writing JavaScript by hand, leveraging jQuery, or configuring Google Tag Manager, there are accessible ways to optimize your site’s performance—without giving up essential integrations.
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