10 ways to Minimize Main Thread Work to Improve Core Web Vitals

9 Jul 2024 | 14 min read
10 ways to Minimize Main Thread Work to Improve Core Web Vitals

What is Main Thread Work?

Main thread work refers to the tasks that the browser’s main thread handles to make a web page work properly. Think of the main thread as the browser’s control center. It does important jobs like:

  • Processing HTML and CSS: Building the page structure and style.
  • Running JavaScript: Adding interactive features and fetching data.
  • Rendering: Drawing the page on the screen and handling animations.
  • Handling User Actions: Responding to clicks, taps, and scrolls.

When the main thread is busy, the website can feel slow and unresponsive. Reducing the work on the main thread helps make websites faster and more user-friendly. This is important for a better user experience and improved performance metrics like Core Web Vitals.

Why is Minimizing Main Thread Work Important?

Minimizing main thread work is crucial for improving the performance and user experience of a website. When the main thread is overloaded, it struggles to keep up with tasks like rendering the page, running scripts, and responding to user interactions. This can make the website feel slow and unresponsive.

Improves User Experience

A smooth and responsive website keeps users happy. When the main thread isn’t bogged down with too many tasks, it can quickly respond to clicks, taps, and other interactions. This makes the website feel faster and more enjoyable to use. Users are more likely to stay on a site that feels responsive and doesn’t lag.

Boosts Performance Metrics

Core Web Vitals are a set of performance metrics that Google uses to measure the quality of user experience on a webpage. These include Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS). When you minimize main thread work, you improve these metrics. For example, less main thread work can lead to a lower FID, meaning the page responds faster to user input.

Enhances SEO

Google considers performance metrics like Core Web Vitals in its search rankings. A website that performs well and offers a good user experience can rank higher in search results. This means more visibility and more traffic to your site. By minimizing main thread work, you not only improve the user experience but also boost your site’s SEO.

Ways to Minimize Main Thread Work 

Optimize JavaScript Execution

Optimizing JavaScript execution is one of the most effective ways to minimize main thread work and improve your website’s Core Web Vitals. JavaScript can significantly impact the performance of a webpage because it requires processing power from the main thread. When you optimize how JavaScript is handled, you reduce the load on the main thread, making your site faster and more responsive.

Code Splitting

Code splitting is a technique that breaks down your JavaScript code into smaller chunks. Instead of loading a large JavaScript file all at once, the browser only loads the code needed for the initial page load. Additional code is loaded as needed. This reduces the initial load time and helps the main thread handle tasks more efficiently. Tools like Webpack can help you implement code splitting in your projects.

Tree Shaking

Tree shaking removes unused code from your JavaScript files. When you import libraries, not all parts of the library might be necessary for your project. Tree shaking identifies and eliminates this unnecessary code, reducing the overall size of your JavaScript files. Smaller files mean less work for the main thread, leading to faster execution and better performance.

Defer Non-Critical JavaScript

Using the async and defer attributes on script tags can defer the loading and execution of non-critical JavaScript. The async attribute allows the script to download in the background while the page continues to load. Once the script is downloaded, it executes immediately. The defer attribute also allows background downloading but ensures the script executes only after the HTML document is fully parsed. This approach prevents JavaScript from blocking the initial rendering of the page, improving load times and reducing main thread work.

Minify JavaScript

Minifying JavaScript involves removing unnecessary characters, such as spaces and comments, without changing its functionality. Minified code is smaller in size, which means the browser can download and execute it faster. Tools like UglifyJS and Terser can automate the minification process, helping you optimize your JavaScript with minimal effort.

Avoid Long-Running JavaScript

Long-running JavaScript tasks can monopolize the main thread, causing delays in rendering and responding to user input. Break down long tasks into smaller, asynchronous tasks using techniques like requestIdleCallback or setTimeout. This approach allows the browser to handle other critical tasks in between, improving overall performance and responsiveness.

Defer Non-Critical JavaScript

Deferring non-critical JavaScript is an effective method to minimize main thread work and improve Core Web Vitals. When you defer JavaScript that isn’t essential for the initial load, you allow the browser to focus on rendering the page quickly and efficiently. This approach helps ensure that users see the content faster and can interact with the page without delay.

Using async and defer Attributes

The async and defer attributes are tools that let you control how and when JavaScript is loaded and executed.

  • Async Attribute: When you add the async attribute to a script tag, the browser downloads the script while it continues to process the HTML. Once the script is downloaded, it executes immediately. This is useful for scripts that are independent and don’t rely on other scripts or the HTML structure.
  • Defer Attribute: The defer attribute works similarly by downloading the script while the HTML is being processed. However, it ensures that the script only executes after the HTML is fully parsed. This makes it ideal for scripts that need to interact with the DOM.

Using these attributes, you can prevent non-critical JavaScript from blocking the rendering of your webpage. For example, analytics scripts, social media widgets, and other third-party integrations are often non-critical and can be deferred. This way, the browser can prioritize rendering the essential content that users need to see first.

By deferring non-critical JavaScript, you reduce the load on the main thread during the initial page load. This leads to faster rendering times and a better user experience. Users can start interacting with the page sooner, and important performance metrics like First Input Delay (FID) and Largest Contentful Paint (LCP) improve.

Reduce JavaScript Payloads

Reducing JavaScript payloads is crucial for minimizing main thread work and improving your website’s Core Web Vitals. Large JavaScript files can slow down your site by increasing load times and adding more work for the main thread to process. By reducing the size of your JavaScript files, you can make your website faster and more responsive.

Minify JavaScript

Minifying JavaScript means removing unnecessary characters like spaces, comments, and line breaks from your code. This process reduces the file size without changing the code’s functionality. Smaller files load faster and put less strain on the main thread. Tools like UglifyJS and Terser can automate this process, making it easy to implement.

Avoid Long-Running JavaScript

Long-running JavaScript tasks can block the main thread, making your site feel slow and unresponsive. Breaking these tasks into smaller, asynchronous chunks can help. Use techniques like requestIdleCallback or setTimeout to split up long tasks. This approach lets the browser handle other important tasks between chunks, improving overall performance.

Optimize CSS

Optimizing CSS is essential for minimizing main thread work and improving Core Web Vitals. CSS (Cascading Style Sheets) determines the look and feel of your website, but if not managed properly, it can significantly slow down the rendering process. By optimizing CSS, you can enhance your site’s performance and provide a better user experience.

Minimize CSS Complexity

Minimizing CSS complexity means simplifying your stylesheets. Large, complex CSS files take longer to process, increasing the load on the main thread. You can reduce complexity by removing unused CSS rules, combining similar styles, and avoiding overly specific selectors. Tools like PurgeCSS can help identify and remove unused CSS, making your files smaller and faster to load.

Critical CSS

Critical CSS involves inlining the CSS needed for the initial view of the page directly in the HTML. This practice ensures that the essential styles are loaded and applied as soon as possible, reducing render-blocking and improving load times. The rest of the CSS can be loaded asynchronously. This approach helps the main thread render the initial content faster, improving metrics like Largest Contentful Paint (LCP).

Use Web Workers

Using Web Workers is a powerful way to minimize main thread work and improve Core Web Vitals. Web Workers allow you to run scripts in background threads, separate from the main thread. This means that heavy computational tasks don’t block the main thread, keeping your website responsive and fast.

Offloading Heavy Computations

Web Workers are ideal for handling tasks that require significant processing power, such as data processing, complex calculations, or handling large datasets. By offloading these tasks to Web Workers, you free up the main thread to focus on rendering the page and responding to user interactions.

For example, if your website needs to process a large amount of data or perform complex calculations, you can move these operations to a Web Worker. The Web Worker performs the tasks in the background, and when it finishes, it sends the results back to the main thread. This way, the main thread remains available to handle user inputs and render updates quickly.

Using Web Workers can significantly enhance the performance of your site, especially for web applications that rely on heavy computations. This leads to a smoother and more responsive user experience, improving metrics like First Input Delay (FID).

Reduce Layout Thrashing

Reducing layout thrashing is essential for improving Core Web Vitals. Layout thrashing happens when the browser has to repeatedly calculate the layout of a page due to changes in the DOM. This can slow down your site and make it less responsive. Here’s how you can reduce layout thrashing:

Avoid Forced Synchronous Layouts

Forced synchronous layouts occur when you read layout properties (like offsetHeight) immediately after modifying the DOM. This forces the browser to recalculate the layout, which takes time and can cause jank. To avoid this, batch your DOM reads and writes separately. For example, read all necessary properties first, then apply all changes afterward. This minimizes the number of layout recalculations.

Use Efficient CSS

Efficient CSS means writing styles that don’t trigger unnecessary layout calculations. Avoid using CSS properties that require intensive calculations, like complex animations or transitions. Simplify your stylesheets and ensure they are optimized to reduce the impact on layout recalculations.

Minimize Layout-Triggering JavaScript

JavaScript that frequently changes the DOM can cause layout thrashing. Minimize these changes by grouping them together or using techniques like requestAnimationFrame to batch updates. This allows the browser to handle the changes more efficiently and reduces the need for repeated layout calculations.

Debounce and Throttle Event Handlers

Event handlers that trigger frequently, like scroll or resize events, can cause layout thrashing if they involve DOM reads and writes. Use debounce and throttle techniques to limit the frequency of these handlers. This ensures that layout recalculations happen less often and the main thread isn’t overloaded.

Use CSS Transforms for Animations

When creating animations, prefer using CSS transforms (like translate, scale, rotate) instead of properties that trigger layout recalculations (like top, left, width, height). CSS transforms are handled by the GPU, which is more efficient and doesn’t cause layout thrashing.

Lazy Load Images and Videos

Lazy loading images and videos is a smart way to minimize main thread work and improve your website’s Core Web Vitals. By deferring the loading of non-essential media, you ensure that the main thread can focus on rendering critical content first. This makes your site load faster and improves user experience.

Deferring Off-Screen Content

Lazy loading involves loading images and videos only when they are about to enter the viewport (the visible part of the web page). Instead of loading all media files as soon as the page loads, lazy loading ensures that only the necessary content is fetched initially. This reduces the initial load time and decreases the workload on the main thread.

For example, if your webpage has several images and videos that are not immediately visible, you can set them to load only when the user scrolls down to that part of the page. This is achieved by using the loading=”lazy” attribute in the HTML img and iframe tags, or by using JavaScript libraries like LazyLoad.

Lazy loading significantly reduces the amount of data the browser needs to process during the initial load. This not only speeds up the page rendering but also helps in conserving the user’s bandwidth, especially important for mobile users. With less data to handle initially, the main thread can render the essential content more quickly, improving metrics like Largest Contentful Paint (LCP).

Optimize Third-Party Scripts

Optimizing third-party scripts is crucial for minimizing main thread work and improving your website’s Core Web Vitals. Third-party scripts, such as ads, analytics, and social media widgets, can significantly impact your site’s performance. By managing these scripts effectively, you can reduce their load on the main thread and enhance user experience.

Minimizing Impact of Third-Party Resources

Third-party scripts often come from external sources and can introduce performance bottlenecks. Here are some strategies to optimize their impact:

Identify Essential Third-Party Scripts: Not all third-party scripts are crucial for your website. Identify which scripts are essential for your site’s functionality and user experience. Remove any scripts that are not providing significant value.

Load Scripts Asynchronously: Use the async or defer attributes to load third-party scripts without blocking the rendering of your page. The async attribute allows the script to download while the HTML continues to parse, executing the script as soon as it finishes downloading. The defer attribute also allows the script to download while parsing but ensures the script executes only after the HTML is fully parsed.

Use Preconnect and DNS Prefetch: Use the rel=”preconnect” and rel=”dns-prefetch” link attributes to establish early connections to third-party domains. This reduces the time it takes to fetch the scripts when they are needed, speeding up the overall load time.

Bundle Third-Party Scripts: Where possible, bundle multiple third-party scripts together. This reduces the number of HTTP requests, which can speed up loading times.

Load Third-Party Scripts Later: Defer the loading of non-critical third-party scripts until after the main content has loaded. This ensures that the critical rendering path is not blocked by scripts that are not immediately needed.

Monitor Script Performance: Continuously monitor the performance impact of third-party scripts. Use tools like Lighthouse or Chrome DevTools to identify slow or blocking scripts and take action to optimize them.

By implementing these strategies, you can significantly reduce the negative impact of third-party scripts on your website’s performance. This optimization helps ensure that the main thread is not overloaded, leading to faster load times and a smoother user experience.

Efficiently Handle Event Listeners

Efficiently handling event listeners is a key strategy to minimize main thread work and improve your website’s Core Web Vitals. Event listeners respond to user actions like clicks, scrolls, and keyboard inputs. If not managed well, they can create performance bottlenecks, slowing down your site. By optimizing how event listeners are used, you can ensure a smoother, more responsive user experience.

Debouncing and Throttling

Debouncing and throttling are techniques to control the rate at which event handlers are executed.

Debouncing: This technique ensures that an event handler is invoked only after a specified period of inactivity. For example, if a user is typing in a search box, debouncing can delay the search function until the user stops typing for a few milliseconds. This reduces the number of times the search function runs, saving main thread resources.

Throttling: Throttling limits the number of times an event handler can be executed within a certain time frame. For example, when a user is scrolling, throttling ensures that the scroll event handler runs at most once every few milliseconds. This prevents the handler from executing too frequently and overloading the main thread.

Passive Event Listeners

Using passive event listeners can improve scrolling performance. When you add a passive event listener, you inform the browser that the event handler will not call preventDefault(). This allows the browser to perform optimizations for better performance.

To add a passive event listener, set the passive option to true:


window.addEventListener('scroll', handleScroll, { passive: true });

Unregister Unnecessary Event Listeners

Remove event listeners that are no longer needed to free up resources. For example, if a particular feature or element is no longer visible or relevant, unregister its event listeners to reduce the load on the main thread.

Optimize Event Handler Code

Ensure that the code inside your event handlers is optimized and efficient. Avoid heavy computations and complex operations within event handlers. If necessary, use techniques like requestAnimationFrame for smoother animations and updates.

Preload Key Requests

Preloading key requests is an effective way to minimize main thread work and improve your website’s Core Web Vitals. Preloading helps ensure that critical resources are available as soon as they are needed, reducing load times and enhancing user experience.

Prioritizing Important Resources

Preloading involves telling the browser to fetch important resources early, before they are needed. This can include CSS files, JavaScript files, fonts, and images that are essential for rendering the initial view of the webpage. By preloading these resources, you can ensure they are available immediately, speeding up the rendering process and reducing the time the main thread spends waiting for these resources.

To preload a resource, you can use the <link rel=”preload”> tag in your HTML. For example, to preload a CSS file, you would add the following line to your HTML <head> section:


<link rel="preload" href="styles.css" as="style">

This tells the browser to fetch the styles.css file early, so it’s ready when needed.

Improving Largest Contentful Paint (LCP)

Preloading key resources can significantly improve the Largest Contentful Paint (LCP) metric. LCP measures how long it takes for the largest visible content element to load on the screen. By preloading the resources required to render this element, you can reduce the LCP time, making the page appear faster to users.

Reducing Render-Blocking

Preloading helps reduce render-blocking, which occurs when the browser has to wait for critical resources to download before it can render the page. When you preload these resources, you ensure they are available when the browser needs them, reducing the time spent on render-blocking tasks.

Using as Attribute

When preloading resources, it’s important to use the correct as attribute value to specify the type of resource being preloaded. This helps the browser prioritize and handle the resource appropriately. For example:

  • Use as=”style” for CSS files.
  • Use as=”script” for JavaScript files.
  • Use as=”font” for web fonts.

Preconnect to Important Origins

In addition to preloading, you can use the rel=”preconnect” attribute to establish early connections to important origins. This reduces the time needed to set up connections when resources are fetched, further speeding up the loading process.

For example:


<link rel="preconnect" href="https://example.com">

In summary, preloading key requests is a powerful technique for improving Core Web Vitals. By prioritizing important resources, reducing render-blocking, and using the correct as attribute, you can ensure a faster, smoother loading experience for users. This leads to better performance metrics and a more responsive website.

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