Improving First Input Delay (FID) is about making your website respond faster to user actions like clicks or taps. FID is one of Google’s Core Web Vitals, and a good score (under 100ms) can boost user experience and search rankings. Long JavaScript tasks, heavy third-party scripts, and unoptimized assets are common causes of poor FID. Here’s how to fix it:
- Optimize JavaScript Execution: Minify, compress, and break up long tasks to free up the browser’s main thread.
- Break Up Long Tasks: Split large JavaScript operations into smaller chunks to prevent blocking user interactions.
- Use Web Workers: Offload heavy computations to Web Workers to keep the main thread responsive.
- Reduce Third-Party Scripts: Limit or optimize scripts like ads and analytics that compete for resources.
- Optimize CSS Delivery: Inline critical CSS and defer non-essential styles to speed up rendering.
- Compress and Minify Assets: Shrink file sizes with tools like GZIP or Brotli for faster downloads.
- Improve Server Response Times: Use CDNs, caching, and faster hosting to reduce delays.
Want a faster, more responsive site? Start by addressing these areas to lower FID and improve user satisfaction.
7 Ways to Improve First Input Delay (FID) – Performance Optimization Guide
How to improve First Input Delay for a better page experience
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1. Optimize JavaScript Execution
JavaScript is often the main culprit behind high FID (First Input Delay) scores. When the browser’s main thread is tied up parsing, compiling, or executing JavaScript, it can’t immediately respond to user interactions like clicks or taps. This delay is noticeable and can frustrate users.
The issue largely stems from long tasks – JavaScript executions that exceed 50 milliseconds. These tasks hog the main thread, creating a bottleneck that blocks the browser from processing user inputs. As MDN Web Docs aptly states:
"The most performant, least blocking JavaScript you can use is JavaScript that you don’t use at all."
To tackle this, start with basic search engine optimization techniques like minifying and compressing JavaScript files. Modern build tools often handle these processes automatically. For more complex scenarios, break up lengthy functions using asynchronous methods like setTimeout() or Scheduler.yield(). These approaches allow the browser to handle user inputs in between tasks.
Impact on FID
Improving JavaScript execution directly affects FID. Even small reductions in script execution times can shift your FID score from "Poor" (over 300ms) to "Good" (under 100ms), where interactions feel nearly instantaneous to users.
Ease of Implementation
Simpler fixes like minifying, compressing, or using async and defer attributes are relatively easy to apply. However, more advanced solutions, such as refactoring synchronous code or leveraging Web Workers, require deeper expertise.
Compatibility with Modern Web Standards
Techniques like requestAnimationFrame and Web Workers are broadly supported across major browsers, making them reliable choices today. Meanwhile, the newer Scheduler.yield() API offers improved task prioritization but may need fallback options for older browsers. To ensure broad compatibility, consider using conditional polyfills that only load for browsers lacking native support. This avoids burdening modern browsers with unnecessary code.
2. Break Up Long Tasks
When JavaScript runs a task that takes more than 50 milliseconds, it blocks the browser’s main thread. This means the browser can’t respond to user interactions during that time. Web performance expert Harlan Wilton puts it this way:
"A click handler can’t fire. A scroll can’t register. A keystroke goes unacknowledged. The user sees a frozen interface."
The fix? Task chunking. This involves breaking down large JavaScript operations into smaller tasks that each take less than 50 milliseconds. For example, instead of running a single 500-millisecond task, you could divide it into ten smaller tasks. This gives the browser regular chances to handle user inputs, cutting the worst-case delay to under 100 milliseconds. By combining this approach with optimized JavaScript execution, you can significantly reduce First Input Delay (FID) and improve how quickly your site responds to users.
Impact on FID
Chunking tasks into smaller pieces has a clear effect on reducing delays. On average, websites have about 3.5 seconds of total JavaScript execution time. This could result in around 70 instances where interactions are delayed. By breaking up these tasks, you allow the browser to handle inputs more often, potentially improving your FID from "Poor" (over 300 milliseconds) to "Good" (under 100 milliseconds). This method directly helps you achieve a more responsive site and a better user experience.
Ease of Implementation
Modern browsers make this process easier with tools like scheduler.yield(), which lets you pause tasks. For older browsers, you can use setTimeout(callback, 0) combined with isInputPending() checks. Keep in mind, though, that each yield adds roughly 4 milliseconds of overhead, so it’s best to use this technique sparingly.
Compatibility with Modern Web Standards
Breaking up long tasks is supported across all major browsers. APIs such as requestIdleCallback() and Web Workers are widely available for this purpose. Newer tools like scheduler.yield() provide better task prioritization, while the setTimeout() fallback remains effective for older browsers. Modern frameworks like React 18 also offer features like useTransition and useDeferredValue, which help manage updates without blocking the main thread. These tools make it easier to implement task chunking while keeping your site aligned with current web standards.
3. Use Web Workers
Web Workers are a smart way to handle heavy computations without bogging down the main thread. Since JavaScript operates on a single thread, tasks that take longer than 50 milliseconds can freeze the user interface, leading to a poor experience. By offloading these intensive tasks to Web Workers, you ensure the main thread stays available for user interactions, keeping the UI smooth and responsive.
Web Workers use a message-passing API (postMessage) to communicate with the main thread. This design keeps them isolated from the DOM, making them perfect for resource-heavy operations like sorting massive arrays, processing complex JSON, manipulating images, or performing cryptographic calculations. For instance, transferring a 10MB buffer using Transferable Objects takes only microseconds, avoiding the typical 20–50ms delay on the main thread.
Impact on FID
Shifting tasks over 50 milliseconds to a Web Worker can make a noticeable difference in reducing First Input Delay (FID). With the average website spending about 3.5 seconds on JavaScript execution, there’s plenty of room for the main thread to get blocked. Offloading major computations – like client-side search indexing or generating thumbnails – frees up the main thread, allowing it to respond to user input instantly. This can push FID from "Poor" (over 300 milliseconds) to "Good" (under 100 milliseconds).
Ease of Implementation
Modern tools like Vite or Webpack 5 make integrating Web Workers straightforward, often requiring just a simple import. Libraries like Comlink can further streamline the process by transforming the usual postMessage calls into cleaner, asynchronous function calls. To get started, use Chrome DevTools to profile your site and pinpoint tasks that could benefit from being offloaded. Keep in mind, though, that for very short tasks, the overhead of message passing might cancel out the gains.
A good practice is to initialize workers during idle times after the page loads and reuse them instead of creating new ones for every task. This approach complements other optimization strategies and helps maintain consistently low FID scores.
Compatibility with Modern Web Standards
Dedicated Web Workers, along with their message-passing API, have been supported by all major browsers since 2012, including Internet Explorer 10. Advanced features like SharedArrayBuffer are available in Chrome 68+, Firefox 79+, and Safari 15.2+ (though they require Cross-Origin Isolation headers). Tools like Partytown can even move resource-heavy third-party scripts – like those from ads, analytics, or chat widgets – into workers automatically, with no code changes needed. Just remember to limit the number of workers to navigator.hardwareConcurrency - 1 to avoid overloading your CPU cores.
4. Reduce Third-Party Scripts
Cutting down the impact of third-party scripts is a crucial step if you’re aiming for an FID (First Input Delay) of 100ms or less. These scripts – like those from analytics tools, chat widgets, or ad networks – often account for 30–50% of a page’s load time. The problem? They compete for the browser’s main thread, delaying responses to user actions like clicks or taps. This tug-of-war for resources directly increases FID, making interactions feel sluggish.
Impact on FID
Here’s an example: YouTube embeds can block the main thread for anywhere between 1.6 and 4.5 seconds. Similarly, tools like Zendesk download around 500KB of JavaScript (2.3MB unzipped), and Drift adds another 200–400KB. These scripts, when executed synchronously, essentially "freeze" user interactions until they finish loading.
Instacart faced this challenge in 2024 and took a bold approach by using Cloudflare Zaraz to shift their third-party tools to server-side execution. The results were striking: Total Blocking Time dropped from 500ms to 0ms, Time to Interactive improved by 63% (from 11.8 seconds to 4.26 seconds), and CPU time was cut by 60% (from 3.62 seconds to 1.45 seconds).
Ease of Implementation
The first step? Run a Lighthouse audit to pinpoint scripts that block the main thread for more than 250ms. Once identified, remove any non-essential scripts and optimize the ones you decide to keep. Use async for scripts that can run independently (like analytics) and defer for those that rely on the DOM. For heavier widgets, such as video embeds or chat boxes, implement lightweight placeholders that load the full script only when users interact with them.
Chrome DevTools’ "Request Blocking" feature is another handy tool. It lets you temporarily disable specific third-party domains to see their true impact on performance. Additionally, use <link rel="preconnect"> for critical third-party domains to speed up DNS lookups and TLS negotiations, saving valuable milliseconds. You can also explore tools like Partytown to offload scripts to Web Workers. For instance, moving Google Tag Manager to a worker can reduce Total Blocking Time by an impressive 92%.
5. Optimize CSS Delivery
CSS can delay rendering because browsers need to download and process stylesheets before displaying content. When CSS files are large or unoptimized, they occupy the main thread and slow down user interactions. By refining how CSS is delivered, you can significantly reduce these delays. Minimizing render-blocking CSS is crucial for achieving a First Input Delay (FID) under 100ms. Let’s explore how CSS optimizations affect FID, their ease of implementation, and their alignment with modern web standards.
Impact on FID
Streamlining CSS works similarly to script optimizations: it frees up the main thread, speeding up FID. Minifying CSS shrinks file sizes by removing unnecessary characters. Combining minification with Brotli or GZIP compression further reduces download and parsing times. Inlining critical CSS eliminates additional network requests, allowing browsers to render visible content faster and make the page interactive sooner. These strategies can improve Largest Contentful Paint (LCP) by 35–60% and cut First Contentful Paint (FCP) by up to 45%.
For example, in 2025, developer RossLab optimized a financial tracking app by inlining critical CSS and prioritizing resources. This reduced FID from 280ms to 45ms, an 84% improvement, and brought LCP down from 4.2 seconds to 1.8 seconds.
Ease of Implementation
To optimize CSS delivery, focus on extracting and inlining above-the-fold styles, keeping them under 14KB compressed – the typical limit for a single TCP roundtrip. Tools like Critical, Penthouse, or Beasties can automate this process. For non-essential CSS, use the media="print" swap pattern:
<link rel="stylesheet" href="..." media="print" onload="this.media='all'"> This method loads stylesheets at a lower priority without blocking rendering. You can also audit frameworks like Bootstrap or Foundation to remove 60–80% of unused CSS. Additionally, setting "Expires" headers – 24 hours for CSS or up to a year for static assets – helps leverage browser caching.
Compatibility with Modern Web Standards
These CSS delivery techniques align with current web standards and are widely recognized as best practices. The media="print" swap pattern is now the preferred method for asynchronous stylesheet loading. Moreover, the content-visibility: auto property, which became widely supported in 2024, can significantly reduce rendering time (by 45–90%) for content-heavy pages. However, avoid using it on LCP elements, as it may delay their appearance. This property works seamlessly across major browsers, including Chrome, Firefox, Edge, and Safari.
6. Compress and Minify Assets
After optimizing tasks, the next step is to reduce file sizes, which helps free up the main thread even more.
Impact on FID
Every byte downloaded uses up the main thread’s resources. For instance, if a user clicks a button while the browser is still processing a large JavaScript file, that interaction gets delayed, waiting for the CPU to catch up. Minification removes unnecessary elements like spaces, line breaks, and comments from HTML, CSS, and JavaScript files without altering their functionality. Compression methods, such as GZIP or Brotli, then encode those files into smaller sizes, making them quicker to transfer.
Take jquery.js as an example: minifying it reduces its size from 260KB to just 32KB, an 87.7% decrease. Smaller files mean faster downloads and quicker execution, freeing up the main thread to handle user interactions. Since FID measures the delay before the browser can respond to the first user action, reducing file size directly lowers this delay. Minifying JavaScript often yields the most dramatic results, with 30–60% size reductions achieved through techniques like variable renaming and removing unused code.
Ease of Implementation
Modern tools make asset optimization a breeze. For instance, esbuild is significantly faster than older JavaScript minifiers like Terser, running 10–100 times faster – perfect for large projects. WordPress users can rely on plugins like WP Rocket, which automate the process of minifying HTML, CSS, and JavaScript with minimal setup. Online tools like purifycss.online can also help by automatically stripping out unused CSS code.
Setting up server-side compression is straightforward, too. Both NGINX and Apache servers can easily be configured to apply Brotli or GZIP compression to text-based assets. For best results, opt for Brotli, which compresses text files 15–20% better than GZIP and is supported by all modern browsers. Pairing this with content hashing in your build process ensures users get updated files when needed, while still benefiting from aggressive caching. This streamlined process works hand in hand with earlier JavaScript optimizations.
Compatibility with Modern Web Standards
Both GZIP and Brotli are widely supported by all major browsers. These compression techniques integrate seamlessly with tools like Webpack, Vite, and Parcel, making them easy to adopt in modern workflows. When minifying files, remember to generate source maps. These maps make debugging easier without exposing them to users, giving you optimized production code alongside readable files for development.
"The less assets there are to download and process, the faster a browser can parse and paint and be interactive to the user." – Portent
Before deploying, test your minified assets. While issues are rare, automated tools can sometimes introduce bugs, especially with complex JavaScript. Use the Chrome DevTools Coverage Tab to find and remove unused code that could unnecessarily block the main thread.
7. Improve Server Response Times
Improving server response times addresses delays right at the source, ensuring a smoother user experience.
Impact on FID
Server response time, measured as Time to First Byte (TTFB), plays a crucial role in how quickly your browser receives the first byte of data after making a request. If the server is slow, it delays essential processes like HTML parsing, script loading, and code execution. This can cause the main thread to stay occupied with heavy tasks, which might coincide with a user’s first interaction, leading to a higher First Input Delay (FID).
Google Lighthouse identifies server response times exceeding 600 ms as a red flag. Since the target FID is 100 ms or less, slow server responses can create a chain reaction that disrupts timely task execution.
Ease of Implementation
Fortunately, there are several practical ways to cut down server response times. Here are a few effective strategies:
- Use a Content Delivery Network (CDN): Services like Cloudflare, Akamai, or Fastly store your content on edge servers closer to users, reducing delays caused by network distance.
- Implement Server Caching: Tools like Varnish, Redis, or Memcached store pre-rendered HTML, so pages don’t need to be built from scratch with every request.
- Upgrade Hosting: Moving from shared hosting to a Virtual Private Server (VPS) or cloud hosting ensures dedicated CPU and memory resources, improving performance.
- Optimize Databases: For sites relying heavily on databases, adding indexes to frequently queried tables or consolidating queries can significantly improve retrieval speeds.
- Update PHP Versions: If you’re using WordPress, upgrading to the latest PHP version often delivers immediate performance improvements.
These adjustments are relatively simple to implement and can make a noticeable difference.
Compatibility with Modern Web Standards
Modern web protocols and techniques can further enhance server performance:
- HTTP/3: This protocol reduces connection setup times, though it doesn’t directly impact internal server processing speeds.
- Streaming for React Apps: Using
renderToNodeStream()in React allows parts of the page to become interactive before the entire response is complete, speeding up user interactions. - Compression: Tools like Brotli and GZIP compress text-based assets before transmission, reducing load times.
However, some methods, like Server-Side Rendering (SSR), require careful consideration. While SSR can make content visible faster, it might increase FID if server-side logic is slow or client-side rehydration creates long tasks that block the main thread. Testing server response times with Lighthouse is essential to ensure they stay under 600 ms.
Conclusion
Improving First Input Delay (FID) is all about creating a smoother, faster experience for your users. A well-thought-out approach to FID optimization can make a lasting difference in how people perceive and interact with your site.
"The first input delay will be the user’s first impression of your site’s responsiveness, and first impressions are critical in shaping our overall impression of a site’s quality and reliability." – Philip Walton, Engineer, Google
Since FID measures real-world interactions, consistent monitoring is essential. The goal? At least 75% of your users should experience a FID of 100ms or less. Tools like Core Web Vitals can help you track progress and spot issues before they affect your rankings or user retention.
Meeting Core Web Vitals thresholds isn’t just about technical performance – it’s about keeping users engaged. Sites that hit these benchmarks see a 24% lower chance of users abandoning page loads. And because FID became a part of Google’s Page Experience signals in 2021, maintaining a low FID score also boosts your SEO. To stay ahead, set performance budgets, audit third-party scripts, and test regularly to avoid slowdowns.
Need expert help? Upward Engine specializes in web performance solutions. From identifying bottlenecks to implementing fixes, we’ll help you maintain top-notch FID and deliver the kind of user experience that keeps visitors coming back.
FAQs
How do I measure my site’s FID?
To gauge your site’s First Input Delay (FID), focus on analyzing the 75th percentile of page loads. This approach helps capture a more typical user experience rather than just the best or worst cases. Since FID is all about real user interactions, it’s essential to use tools that monitor actual user behavior. These tools measure the time it takes for the browser to respond after a user interacts with the page, offering an accurate FID score based on real-world data.
What’s the fastest fix if my FID is high?
To quickly improve a high FID (First Input Delay), focus on reducing and optimizing your JavaScript payloads. Begin by splitting your code and prioritizing the loading of only essential scripts during the initial load. This approach minimizes main thread blocking time, leading to faster responsiveness and a smoother user experience.
Should I focus on FID or INP now?
With the latest updates to web performance metrics, the focus has shifted from FID (First Input Delay) to INP (Interaction to Next Paint). This change took effect in March 2024, making INP a Core Web Vital. Unlike FID, which only measured the delay of the first user interaction, INP evaluates responsiveness across all interactions on a page.
Why does this matter? Optimizing for INP ensures your site meets modern performance standards while enhancing the overall user experience. This is particularly important since many websites still fall short of meeting the recommended thresholds for responsiveness. By prioritizing INP, you’re not just keeping up with the latest metrics – you’re creating a smoother, more enjoyable experience for your users.
