What Impact Do Third-Party Tags and Scripts Have on Core Web Vitals, and How Can I Optimize Their Loading for Better Scores?


Third-party tags and scripts can significantly impact Core Web Vitals, primarily by causing increased loading times and latency, which can negatively affect a site's perceived performance. Optimization techniques involve deferring non-critical scripts, loading asynchronously, managing tag priority, and employing tag management systems.

Effects of Third-party Tags and Scripts on Core Web Vitals

Third-party tags and scripts, such as analytics, A/B testing tools, and advertisements, can profoundly impact [Core Web Vitals, Google] because they add to the amount of code the browser needs to process, which can increase loading times, delay interactivity, and cause layout shifts.

Impact on Specific Metrics

Largest Contentful Paint (LCP)

Third-party scripts loading large resources like images or video can significantly delay the LCP metric, which focuses on loading performance. They can increase load times and cause page rendering slowdowns [Largest Contentful Paint (LCP), Google].

First Input Delay (FID)

Scripts executing during user interaction can delay event handling, impacting the FID metric, which measures interactivity [First Input Delay (FID), Google].

Cumulative Layout Shift (CLS)

Scripts that add or modify content can cause layout shifts, influencing the CLS metric, which measures visual stability [Cumulative Layout Shift (CLS), Google].

Strategies to Optimize Third-party Scripts

Deferring Non-critical Scripts

By deferring non-critical scripts, the browser can first complete the loading and rendering of critical content. Using <script defer> is a commonly recommended practice in these cases [Efficiently load third-party JavaScript, Google].

Async Loading

The async attribute allows the browser to continue parsing the HTML while the script loads, without blocking the render. Scripts that are not essential during the initial page load should be loaded asynchronously with <script async> [The Script Async Attribute, HTML.com].

Manage Tag Priority

Not all third-party scripts are equal; deciding and setting a load priority based on the script importance can greatly enhance page performance [The Script Element, MDN Web Docs].

Use a Tag Management System

A tag management system allows you to manage and deploy third-party tags from a single point, often providing options for prioritization and conditional loading [Google Tag Manager, Google Marketing Platform].


Properly managing third-party tags and scripts can significantly improve Core Web Vitals. Prioritizing critical scripts, deferring or asynchronously loading non-critical scripts, and utilizing a tag management system are recommended practices to optimize their loading for better performance scores.