How Can Asynchronous Loading of Non-Critical JavaScript and CSS Files Improve First Input Delay (FID)?


Asynchronous loading of non-critical JavaScript and CSS files can significantly improve First Input Delay (FID). By loading these assets asynchronously, the browser can prioritize rendering the main content and executing critical tasks, leading to a more responsive user experience. Here’s a deep dive into why this technique is effective and how to implement it.

Understanding First Input Delay (FID)

First Input Delay (FID) is a user-centric metric that measures the time from when a user first interacts with your site (e.g., when they click a link, tap a button, or use a custom JavaScript-powered control) to the time when the browser is able to begin processing that interaction. Improving FID is crucial because a noticeable delay in response can severely impact user experience.

Benefits of Asynchronous Loading

Improving Perceived Load Time

Asynchronous loading of non-critical JavaScript and CSS files allows the browser to render the main content without waiting for these resources to download and execute. This can greatly improve perceived load time for users, as they can interact with the page more quickly.

Reducing Main Thread Workload

Loading scripts asynchronously or deferring their execution can significantly reduce the workload on the main thread, which is responsible for handling user interactions. When the main thread is less busy, it can process user inputs faster, reducing FID.

Implementing Asynchronous Loading

JavaScript: Use async and defer

Both async and defer attributes can be added to <script> tags in HTML to load non-critical JavaScript files asynchronously:

  • async: The script is executed as soon as it is downloaded without blocking the rendering of the page.
  • defer: The script is downloaded in parallel with other resources but is executed only after the HTML parsing has been completed.


<script src="non-critical.js" async></script>
<script src="another-non-critical.js" defer></script>

For a deeper understanding, refer to [Defer Non-Critical JavaScript, 2023](

CSS: Load CSS Asynchronously

Loading CSS asynchronously can also be beneficial in reducing render-blocking resources. This can be done using the media attribute or by dynamically injecting <link> tags through JavaScript:

Example using media attribute:

<link rel="stylesheet" href="non-critical.css" media="print" onload="'all'">

Example using JavaScript:

var link = document.createElement("link");
link.href = "non-critical.css";
link.rel = "stylesheet";

For more details, see [Optimize CSS Delivery, 2022]( Additionally, the [ guide on async CSS]( offers comprehensive insights.

Case Studies and Examples

Case Study: HubSpot

HubSpot implemented asynchronous loading for non-critical assets and saw a significant reduction in their First Input Delay. By deferring non-essential JavaScript and CSS, they enhanced their website's responsiveness, ultimately improving user interaction rates. Learn more from their detailed case study at [HubSpot Case Study, 2021](

Example: eCommerce Sites

Many eCommerce sites use asynchronous loading for tracking scripts, recommendation engines, and other non-essential functionalities to ensure the main shopping experience remains responsive even while these scripts load in the background. This strategy has been shown to increase conversion rates as users experience faster and more seamless interactions.


Asynchronous loading of non-critical JavaScript and CSS files can significantly improve First Input Delay by allowing the browser to prioritize rendering primary content and handling critical tasks. Implementing async and defer attributes for scripts and asynchronously loading CSS can reduce render-blocking resources, ultimately enhancing user experience.