What Strategies Can I Employ to Reduce the Impact of Third-Party Scripts on First Input Delay (FID)?


Reducing the impact of third-party scripts on First Input Delay (FID) is crucial for enhancing web performance and user experience. Strategies include deferring non-essential scripts, lazy loading, optimizing script execution and delivery, and limiting the number of third-party requests. Here’s a comprehensive guide to effectively mitigate the negative effects of third-party scripts on FID.

Deferring Non-Essential Scripts

Using <script> with async and defer

Use the async and defer attributes on script tags to control when the browser executes the scripts. The async attribute ensures the script is executed as soon as it’s available, while the defer attribute waits until the document is fully parsed before executing the script, both of which can prevent blocking of the main thread.

[Optimize LCP: Defer JavaScript, 2023]

Post-loading Third-party Scripts

Load third-party scripts only after the main content has rendered and user interactions are possible. This approach minimizes the impact on FID as critical resources are prioritized.

[Loading Third-Party JavaScript, 2022]

Lazy Loading

Implementing Lazy Loading

Lazy loading defers the loading of non-critical resources until they are needed. This practice can significantly reduce the initial load time and improve FID by reducing the number of script executions during the initial load phase.

[Lazy Loading, 2023]

Optimizing Script Execution

Minifying and Compressing JavaScript Files

Minify and compress JavaScript files to reduce their size. Smaller file sizes lead to faster downloads and reduced blocking time, thereby minimizing their impact on FID.

[Minify JavaScript, 2022]

Reducing JavaScript Execution Time

Identify and remove unnecessary JavaScript, and optimize the performance of remaining scripts. Profiling tools can help find bottlenecks and optimize script execution time.

[Efficiently Load Third-Party JavaScript, 2023]

Limiting Third-Party Requests

Avoiding Redundant Third-Party Scripts

Audit and consolidate third-party scripts to eliminate redundancy. For instance, using one comprehensive analytics tool rather than multiple can reduce the number of requests and code execution time.

[PageSpeed Insights, 2022]

Using a Content Security Policy (CSP)

A CSP can help control which third-party resources are allowed to load, thereby limiting unnecessary script executions. Implementing a restrictive CSP can prevent unwanted scripts from impacting FID.

[Implementing a Strict CSP, 2023]

Example Implementation

Deferring and Lazy Loading Example

Here’s how you can defer and lazy load third-party scripts:

<!-- Defer Analytics Script -->
<script src="https://example.com/analytics.js" defer></script>

<!-- Lazy Load Social Media Widget -->
  if ('IntersectionObserver' in window) {
    document.addEventListener('DOMContentLoaded', function() {
      var observer = new IntersectionObserver(function(entries) {
        entries.forEach(function(entry) {
          if (entry.isIntersecting) {
            var script = document.createElement('script');
            script.src = 'https://example.com/social-widget.js';
<div id="social-widget"></div>


By deferring non-critical scripts, lazy loading resources, optimizing script execution, and limiting the number of third-party requests, you can significantly mitigate the impact of third-party scripts on First Input Delay (FID). Implementing these strategies ensures your website remains responsive and provides a better user experience.