How to Optimize JavaScript for Core Web Vitals

Website performance is critical for user experience and search rankings. Google’s Core Web Vitals focus on three key metrics: Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). Poor JavaScript optimization can harm these metrics, leading to slower load times, unresponsive pages, and lower search rankings.

Here’s the bottom line:

• LCP should load in under 2.5 seconds.
• INP should respond within 200ms.
• CLS should stay below 0.1.

To meet these benchmarks, focus on:

1. Code Splitting: Break large files into smaller chunks.
2. Defer and Async Loading: Prevent scripts from blocking rendering.
3. Minify and Compress: Remove unused code and reduce file sizes.
4. Progressive Hydration: Prioritize interactivity for visible content.
5. Web Workers: Offload heavy tasks to background threads.

These strategies improve user experience, boost conversions, and help your site rank higher. Keep monitoring performance with tools like Lighthouse, Chrome DevTools, and RUM platforms to ensure lasting results.

How to Reduce Unused Javascript | Core Web Vital Masterclass | Part 8

How to Measure JavaScript Performance for Core Web Vitals

Measuring JavaScript performance involves using the right tools and focusing on specific metrics to identify areas for improvement.

Performance Diagnostic Tools

Chrome DevTools’ Performance panel is a great starting point. It allows you to record page loads and pinpoint JavaScript tasks that block the main thread. Similarly, Lighthouse provides in-depth audits to identify JavaScript-related issues that impact Core Web Vitals. You can access Lighthouse through Chrome DevTools, as a Chrome extension, or via the command line for automated testing.

Another valuable tool is PageSpeed Insights, which combines Lighthouse’s lab data with real-user data from the Chrome User Experience Report (CrUX). For broader insights, Google Search Console groups similar pages, making it easier to spot patterns in performance issues.

Here’s the key difference between these tools: lab tools simulate conditions to help troubleshoot problems, while field tools reflect the actual experience of users. Experts often recommend prioritizing field data for a more accurate understanding of performance.

For ongoing tracking, platforms like SearchX offer technical SEO tools to monitor JavaScript performance and quickly address emerging issues.

Once you’ve identified problem areas, it’s time to measure their impact using specific performance metrics.

JavaScript Performance Metrics

Focusing on the right metrics ensures your optimization efforts are targeted. One critical metric is Total Blocking Time (TBT), which measures how long JavaScript tasks block the main thread. Tasks exceeding 50 ms contribute to TBT, and keeping it under 200 ms is essential for good performance.

Long Tasks – those lasting over 50 ms – can delay browser responsiveness, while Script Coverage shows how much of your JavaScript code is actually being used. Unused code not only wastes bandwidth but also increases processing time, potentially raising TBT. Chrome DevTools’ Coverage tab highlights unused portions of your scripts.

Another key metric is Main Thread Blocking Time, which identifies periods when JavaScript prevents other critical tasks from running. This can lead to issues like unresponsive buttons or forms.

TBT is measured between First Contentful Paint (FCP) and Time to Interactive (TTI), making this window a prime opportunity for optimization.

How to Find JavaScript Bottlenecks

Once you’ve identified your metrics, the next step is to locate the specific JavaScript functions and files causing performance issues. Start by using Chrome DevTools’ Performance panel. Enable Screenshots and Web Vitals in the settings, then record a page load to generate a detailed performance timeline.

Look for red blocks in the timeline – these indicate long tasks contributing to TBT. Clicking on a red block reveals task duration and the JavaScript function responsible, which you can view in the Summary tab.

Running a Lighthouse audit can also provide actionable recommendations, such as identifying unused code, suggesting code splitting, or flagging third-party scripts that cause delays. These audits often include estimated savings in milliseconds, helping you prioritize fixes with the most impact.

The Coverage tab in Chrome DevTools is another helpful feature. By loading your page with the Coverage panel active and interacting with your site, you can identify scripts with low usage percentages. These are ideal candidates for code splitting or removal.

Recording a Performance trace during user interactions – like clicking buttons or filling out forms – can highlight operations that delay responsiveness. If blocking times exceed 200 ms, it may negatively affect your Interaction to Next Paint (INP) score.

Don’t overlook third-party scripts. Use the Network panel to identify external scripts that take the longest to load or execute. Consider loading these scripts asynchronously or removing nonessential ones.

For large-scale monitoring, the web-vitals.js library can be integrated with Google Analytics 4 to generate detailed performance reports. This helps track improvements and catch regressions before they affect users.

It’s worth noting that 40% of sites in the Chrome UX Report fail to meet the recommended Largest Contentful Paint (LCP) threshold, often due to JavaScript blocking critical rendering paths. By systematically addressing bottlenecks, you can ensure your site remains fast and responsive for users.

JavaScript Optimization Techniques for Core Web Vitals

Improving Core Web Vitals often boils down to reducing Total Blocking Time (TBT) and enhancing interactivity. By addressing inefficiencies with focused techniques, you can significantly improve your site’s performance.

Code Splitting and Bundle Optimization

Code splitting divides large JavaScript files into smaller, more manageable pieces, reducing the amount of JavaScript that needs to be downloaded, parsed, and executed during the initial page load. This method directly improves your TBT score, which accounts for 30% of the global performance score in Lighthouse. For better results, lazy load non-essential bundles, ensuring only critical JavaScript is loaded upfront.

To implement code splitting effectively, use dynamic imports to load code only when it’s required. Bundle optimization complements this by minimizing file sizes through techniques like minification, compression, and removing unused code. Ben Schwarz, Founder and CEO of Calibre, puts it succinctly:

"Any way you reduce the file size or runtime of your JavaScript will improve your TBT numbers."

Keep in mind that a good TBT score is under 300 milliseconds. After optimizing bundles, focus on script loading attributes to prevent render-blocking JavaScript.

Defer, Async, and Lazy Loading

Once bundles are optimized, ensure non-critical scripts don’t block rendering. Using defer, async, and lazy loading can prevent render-blocking JavaScript and improve Core Web Vitals scores. The async and defer attributes allow browsers to download scripts in the background while rendering the page. However, their execution timing differs:

Use async for scripts that don’t depend on the DOM, like analytics or ad scripts. On the other hand, defer is better for scripts that interact with the DOM or rely on other JavaScript libraries. By placing <script> tags in the <head> with defer, browsers can start loading scripts earlier without interrupting HTML parsing. Lazy loading extends this idea further by delaying the loading of non-critical resources – like images, iframes, or JavaScript modules – until they’re needed.

Minify, Compress, and Remove Unused Code

Unused JavaScript increases page load times and delays rendering metrics like First Contentful Paint (FCP) and Largest Contentful Paint (LCP). It also slows down module evaluation and compilation, negatively impacting interactivity. Tools like Chrome DevTools Coverage, PageSpeed Insights, or DebugBear can help you identify unused JavaScript.

Minification removes unnecessary characters like whitespace and comments, reducing file size. Tools like Google Closure Compiler, UglifyJS, or Terser can automate this process. Combine minification with GZIP compression on the server to further reduce file sizes. To eliminate unused code, use tree shaking and dead code elimination, available in module bundlers like Webpack, Parcel, or Rollup.

Additionally, audit third-party dependencies regularly. Replace heavy external scripts with lighter alternatives to keep your site lean and fast.

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Advanced JavaScript Optimization Methods

When dealing with complex websites and modern frameworks, basic optimization techniques often fall short. Advanced methods can help tackle deeper performance challenges, particularly when aiming to improve Core Web Vitals.

Hydration and Progressive Loading

Hydration refers to the process of attaching client-side JavaScript to server-rendered HTML to make it interactive. Traditional hydration processes the entire application at once, which can create significant performance bottlenecks.

Progressive hydration offers a smarter approach by breaking hydration into smaller, prioritized tasks. Instead of making every component interactive immediately, it focuses on prioritizing tasks based on user interaction, viewport entry, or critical content loading.

For example, progressive hydration has been shown to reduce First Input Delay by over 50%, maxFID by 40%, and bundle size by 30%. Techniques like splitting hydration tasks using setTimeout and dynamic imports have also reduced Total Blocking Time by more than 40% on product pages.

"Progressive hydration shifts the paradigm from ‘hydrate everything’ to ‘hydrate smartly.’ By prioritizing critical components and deferring non-critical ones, we can drastically improve perceived performance while maintaining a rich user experience." – Ignaciojsoler

To implement this effectively, pair progressive hydration with the Intersection Observer API to hydrate components only when they become visible. Frameworks like Astro take this a step further with an "islands" approach, where only interactive sections are hydrated while the rest remains static. For low-priority components, combining islands architecture with requestIdleCallback can optimize performance even further.

Partial hydration goes beyond this by skipping hydration for non-interactive elements altogether. This method has been particularly effective for content-heavy sites, reducing hydration time on landing pages by as much as 80%.

Using Web Workers for Heavy Tasks

JavaScript’s single-threaded nature means that intensive operations can block the main thread, causing UI lag and a poor user experience. Web Workers solve this issue by running scripts in background threads, offloading heavy tasks from the main thread. This keeps the interface smooth and responsive.

To use a Web Worker, create a separate JavaScript file for the worker’s code. Use worker.postMessage(data) to send data and set up worker.onmessage = (e) => {} to handle responses. Always remember to terminate workers with worker.terminate() when they’re no longer needed to avoid memory leaks.

For best results:

• Minimize message size and frequency between threads.
• Use transferable objects like ArrayBuffer for large binary data to reduce copying overhead.
• Consider libraries like comlink to simplify communication with workers.
• Keep worker scripts lightweight and use worker pools to manage multiple tasks efficiently.

It’s important to note that Web Workers don’t have access to the DOM, making them ideal for tasks like CPU-intensive calculations, data processing, and complex algorithms.

Framework-Specific Optimizations

Modern frameworks come with built-in tools to enhance performance – especially important given that 98.7% of websites rely on JavaScript in some capacity.

React has introduced several features aimed at reducing client-side JavaScript:

• React Server Components render components server-side, reducing the need for client-side processing.
• The Actions API simplifies server interactions.
• The React Compiler automates memoization to optimize performance.

Angular has improved state management with Signals, which cuts down on unnecessary re-renders. Its deferrable views allow components to load only when needed, and NgOptimizedImage directly enhances image handling for better Largest Contentful Paint scores.

Vue.js has refined its reactivity system in version 3.4, reducing unnecessary updates. Vue Vapor Mode further optimizes compilation to minimize runtime overhead.

Other frameworks focus on alternative approaches:

• Svelte compiles components into optimized JavaScript during the build process, eliminating the virtual DOM overhead.
• SolidJS updates the real DOM directly for exceptional performance.
• Qwik introduces "resumability", allowing applications to pick up from a server-rendered state without re-executing code.

Here’s a quick comparison of these frameworks and their strengths:

Monitoring and Maintaining JavaScript Performance

Keeping JavaScript performance in check isn’t a one-time task – it’s an ongoing process. Without consistent monitoring, even the best optimizations can backslide with new deployments, undoing months of hard work.

Real User Monitoring (RUM) Tools

Real User Monitoring (RUM) tools let you see how real users experience your site, offering insights that go beyond synthetic testing. These tools track performance across devices, browsers, and networks while monitoring Core Web Vitals like Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). This data helps identify bottlenecks that could hurt user experience and conversions.

Hotplate, for example, faced recurring performance problems but struggled to measure the impact. Rangaraj Tirumala, Founding Engineer at Hotplate, described their challenge:

"We experienced frequent performance issues but lacked the tools to measure the extent of the degradation accurately. It was becoming increasingly harder to react to issues or, more importantly, be aware that issues were going to happen before they occurred."

RUM tools solve this by continuously monitoring real-world performance. For instance, you can use them to:

• Spot slow-loading elements by analyzing LCP across different environments.
• Detect interaction delays by tracking INP scores.
• Identify visual instability by monitoring CLS, allowing you to address issues like missing image dimensions or unreserved ad space.

RUM dashboards provide detailed insights, including page titles, URLs, devices, browsers, user locations, and Core Web Vitals scores. These insights naturally feed into regular performance reviews, helping teams stay proactive.

Regular Performance Reviews

Quarterly performance audits are a great way to ensure your site stays fast and user-friendly. These reviews help you measure progress, identify areas for improvement, and refine your strategies.

Forming cross-functional "Speed Teams" that include members from development, marketing, design, and product management can keep optimization efforts aligned across your organization. During these reviews, focus on:

• Changes in bundle sizes.
• The performance of third-party scripts.
• Framework updates.
• Mobile performance, as mobile users often face slower devices and connections.

It’s also a good idea to set performance budgets and integrate testing into Pull Requests. This way, you can catch issues early before they affect users. With Google factoring Core Web Vitals into rankings, these reviews are essential for both user satisfaction and SEO.

Using SearchX for Continuous Performance Tracking

While internal reviews are crucial, external tools like SearchX can add an extra layer of monitoring. SearchX’s technical SEO services offer advanced tracking and actionable insights for JavaScript performance.

For example, SearchX can:

• Track CLS for every visitor and identify which elements cause layout shifts.
• Pinpoint slow-loading elements and blocking assets to improve LCP.
• Continuously measure INP scores and collect performance timing data to guide optimizations.

Tyler Spencewood, Senior Software Engineer at Nomics, highlighted the value of such tools:

"Request Metrics gives us a comprehensive view of our Core Web Vital scores with clear and detailed metrics across all pages."

SearchX also provides geographic performance data, helping you understand how regional differences in CDN coverage and network conditions affect user experience. Additionally, its integration with broader SEO solutions allows you to link JavaScript performance improvements to key business outcomes like bounce rates, conversions, and search visibility.

Conclusion and Key Takeaways

Optimizing JavaScript for Core Web Vitals isn’t just a technical tweak – it’s a direct line to better user experiences, higher conversion rates, and improved search rankings. With JavaScript powering a staggering 98.3% of websites, ensuring it functions efficiently is critical for protecting your business’s visibility and revenue.

The numbers tell a clear story. By 2025, only 47% of websites are expected to pass Core Web Vitals. And when INP replaced FID in March 2024, nearly 600,000 websites failed to meet the new standards. For those willing to prioritize optimization, this presents a massive opportunity to outperform competitors.

A solid strategy involves multiple layers of improvement. Start with code splitting and bundle optimization to keep file sizes manageable. Use techniques like defer, async, and lazy loading to prevent render-blocking. Minify, compress, and eliminate unused code for immediate performance boosts.

For more complex interactions, advanced methods are key. Progressive loading and hydration ensure responsive pages, while web workers handle heavy computations without slowing down the user experience.

These efforts have a direct impact on your bottom line. As Barry Pollard wisely states:

"Weigh the balance between monetization and performance. If users are dropping off earlier due to a poor experience, then those extra ads may be costing you more revenue than they are adding".

But optimization isn’t a one-and-done deal. Regular reviews and ongoing monitoring are essential to maintaining these gains. Pages that deliver fast, smooth experiences are favored by both users and Google.

Technical SEO remains a cornerstone of meeting Core Web Vitals requirements. To stay ahead of ever-changing standards, consider working with SearchX’s technical SEO services. The goals are straightforward but crucial: achieve LCP within 2.5 seconds, keep INP under 200 milliseconds, and maintain CLS below 0.1. Meeting these benchmarks can mean the difference between frustrated users leaving your site and engaged visitors who convert.

FAQs

What is code splitting, and how does it improve JavaScript performance for Core Web Vitals?

Code splitting is a method used to divide large JavaScript files into smaller, bite-sized pieces. These smaller chunks are only loaded when they’re actually needed, rather than all at once. The result? Faster initial load times and reduced Total Blocking Time (TBT).

By cutting down on the amount of JavaScript processed right away, code splitting makes your site more responsive and improves the overall user experience. It also plays a key role in boosting Core Web Vitals, particularly metrics like First Input Delay (FID) and TBT, which measure how quickly a page reacts to user actions.

What’s the difference between defer, async, and lazy loading, and when should you use each?

defer, async, and lazy loading are powerful tools to enhance how quickly your website loads by optimizing JavaScript and other resources. Here’s a breakdown of what they do and when they’re most useful:

• defer: When you add defer to a script, it downloads in the background while the HTML is being parsed. However, the script won’t run until the HTML is fully loaded, and it executes in the order it appears. This is ideal for scripts that need the DOM to be fully ready before running.
• async: Scripts with async are downloaded and executed as soon as they’re ready, without waiting for the HTML to finish parsing and without preserving the order of execution. This works best for scripts that are self-contained and don’t rely on other scripts or the DOM.
• Lazy loading: This technique delays loading non-critical resources, like images or videos, until they’re actually needed – typically when the user scrolls to them. It’s a great way to prioritize the loading of essential content and improve the initial user experience.

By using these techniques thoughtfully, you can reduce Total Blocking Time and make your site more interactive, which is key for hitting Core Web Vitals targets.

How do Real User Monitoring (RUM) tools improve JavaScript performance for Core Web Vitals?

Real User Monitoring (RUM) tools focus on improving JavaScript performance by collecting data directly from users as they interact with your website. This real-world feedback helps pinpoint problems, such as long Total Blocking Time (TBT) and delays in interactivity, which can hurt your Core Web Vitals scores.

With this data, RUM tools offer practical insights to fine-tune JavaScript execution, cut down load times, and enhance the user experience. The result? A website that aligns with Core Web Vitals standards, delivering better performance and keeping users engaged.

Related posts

• Core Web Vitals as a Ranking Factor Explained
• Core Web Vitals Metrics Explained: CLS, LCP, INP
• Core Web Vitals: Reducing JavaScript Execution Time
• Improving Core Web Vitals With Rendering Optimization