First Input Delay Explained: What You Need to Know


May 2024
First Input Delay Explained

The responsiveness of a website, as measured by First Input Delay (FID), is a critical factor in delivering a positive user experience and maintaining a competitive edge online. FID measures the time from when a user first interacts with your page—whether through clicking a link, tapping a button, or entering text—to when the browser is able to respond to that interaction. As a key component of Google’s Core Web Vitals, The importance of optimizing FID cannot be overstated. It directly influences how users perceive your site’s responsiveness, which can affect engagement rates, conversion rates, and ultimately, your site’s ranking in search engines. A low FID helps ensure that your site feels more responsive and interactive, providing a smooth and seamless user experience. This guide will delve into the importance of FID, methods for measuring and improving it, and strategies to ensure your site offers a responsive and engaging user experience. 

What is First Input Delay?

First Input Delay (FID) is a pivotal performance metric that sheds light on the responsiveness of a website during a user’s initial interaction. As a component of Google’s Core Web Vitals, FID measures the time from when a user first interacts with a page—like clicking a link or entering text—to when the browser is actually able to respond. This metric is critical because it quantifies the ‘real-user’ interactive experience, which is essential for assessing a site’s usability. By improving FID, websites can ensure that users experience minimal delay, enhancing perceived responsiveness and smoothness of the site.

Furthermore, FID’s importance extends into areas like search engine optimization (SEO) since Google integrates Core Web Vitals, including FID, into its ranking criteria. A lower FID, indicating a faster and more responsive site, can lead to enhanced user engagement, reduced bounce rates, and higher SEO rankings. Understanding how FID interacts with other performance metrics, such as Largest Contentful Paint (LCP) and Cumulative Layout Shift (CLS), is crucial. These metrics collectively provide a holistic view of user experience, encompassing load times, interactivity, and visual stability, making FID a key focus for developers aiming to optimize web performance.

Importance of Measuring FID

Measuring FID (First Input Delay) is crucial for assessing the user experience and performance of a website. Here are several reasons why measuring FID is important:

1. User Experience: FID directly impacts user experience. It measures the delay between when a user interacts with a webpage (like clicking a button) and when the browser responds to that interaction. A low FID means users perceive the website as responsive and interactive, leading to higher user satisfaction.

2. Perceived Performance: FID contributes significantly to the perceived performance of a website. Even if the overall loading time is fast, if there’s a delay in responding to user inputs, users may perceive the site as slow and unresponsive.

3. Conversion Rates: Studies have shown that faster websites tend to have higher conversion rates. By optimizing FID and reducing input delay, website owners can improve conversion rates, leading to increased sales, sign-ups, or other desired actions.

4. SEO (Search Engine Optimization): User experience metrics, including FID, are increasingly important factors in search engine rankings. Google, for example, considers page speed and user experience as ranking factors. Optimizing FID can improve SEO performance and increase organic traffic.

5. Engagement and Retention: Websites with fast response times tend to have higher user engagement and retention rates. Users are more likely to stay on a site and explore further if they have a smooth and responsive experience.

6. Competitive Advantage: In today’s competitive online landscape, providing an excellent user experience is essential for staying ahead of competitors. Monitoring and optimizing FID can give websites a competitive advantage by offering a superior user experience.

How to Measure FID

Accurately measuring First Input Delay (FID) is crucial for diagnosing performance issues and improving user interactions on your website. Here’s how you can effectively measure this vital metric:

1. Field Tools for Real User Monitoring: To capture genuine user experiences, utilize tools that measure FID based on real user interactions. The Chrome User Experience Report (CrUX) is an invaluable resource that provides FID metrics from actual site visitors. PageSpeed Insights integrates CrUX data to offer comprehensive reports on FID and other Core Web Vitals. Similarly, Google Search Console’s Core Web Vitals report presents FID data, assisting webmasters in monitoring their site’s performance from a real-world perspective.

2. Client-Side Measurement: For a more hands-on approach during development, the web-vitals JavaScript library enables developers to measure FID directly in the browser. This tool is particularly useful for understanding how different interactions are processed. Additionally, the PerformanceObserver API can be used to monitor and log first input events and their delays, providing deeper insights into what may be causing lags.

3. Analyzing FID Data: After collecting FID data, it’s important to analyze this information to identify patterns or specific issues causing high input delays. Often, prolonged FID times are linked to complex JavaScript tasks or heavy DOM manipulations, which can severely affect the responsiveness of a page. By comparing FID scores across different pages, developers can identify problematic areas and make informed decisions about optimizations.

4. Integrating FID into Performance Budgets: Setting specific targets for FID is a proactive way to maintain site interactivity. Aiming for an FID of 100 milliseconds or less will generally ensure that users perceive your site as responsive. Including FID in your performance budgets—and reviewing these budgets regularly—is a strategic approach to keeping your website’s responsiveness on track.

Factors Affecting FID

Understanding the factors that contribute to First Input Delay (FID) is key to effectively reducing it. Several elements can influence how quickly a page becomes interactive:

1. Heavy JavaScript Execution: JavaScript is often the primary culprit in high FID scores. When browsers process heavy JavaScript tasks, it can delay the response to user inputs. This includes time-consuming parsing, compiling, and executing JavaScript files. Optimizing how JavaScript is loaded and executed on your site is crucial. Techniques such as minimizing and deferring JavaScript, using asynchronous scripts, and removing unused code can significantly improve FID.

2. Complex Web Applications: Web applications that rely heavily on JavaScript frameworks or libraries for rendering content and handling user interactions can also see increased FID times. Single-page applications (SPAs), for example, might delay user inputs while waiting for JavaScript to render content dynamically. Employing server-side rendering (SSR) or static site generation (SSG) can help by sending a fully rendered page to the client, reducing the workload on the browser.

3. Resource Loading Strategies: The order and method in which resources are loaded on a page can affect FID. Resources that block the main thread, such as synchronous scripts or large stylesheets, can prevent the browser from responding quickly to user interactions. Implementing resource loading strategies such as critical resource prioritization, resource hints like preload and prefetch, and ensuring non-critical resources are loaded asynchronously can improve responsiveness.

4. Third-party Scripts: Ads, analytics, and various third-party widgets can impact FID by consuming processing resources and network bandwidth. These scripts often block the main thread or compete with critical resources for bandwidth. Carefully managing third-party scripts by deferring their load, using asynchronous loading, or isolating them in web workers where feasible, can mitigate their impact on FID.

Optimizing First Input Delay

Optimizing First Input Delay (FID) is crucial for enhancing the user experience and ensuring that your website meets the criteria of E-E-A-T (Expertise, Authoritativeness, Trustworthiness).To improve First Input Delay (FID) and enhance site interactivity, developers can implement a range of optimizations. Here are some effective strategies:

1. Optimize JavaScript Loading and Execution: Since JavaScript execution can significantly impact FID, optimizing how JavaScript is handled is crucial. Consider the following techniques:

  • Minimize JavaScript: Reduce the amount of JavaScript needed to load on your site by eliminating unnecessary code and minimizing libraries and frameworks.
  • Use Asynchronous Loading: Apply async or defer attributes to script tags to prevent them from blocking the document’s parsing process.
  • Code Splitting: Implement code splitting to divide your JavaScript into smaller chunks that can be loaded as needed rather than all at once. This approach can significantly reduce the amount of code that needs to be parsed, compiled, and executed on initial load.

2. Enhance Resource Loading: Optimizing how resources are loaded can also improve FID. Strategies include:

  • Prioritize Critical Resources: Ensure that only essential scripts and styles are loaded initially and that less critical resources are loaded after the main content.
  • Implement Resource Hints: Use preload, prefetch, and preconnect to optimize the loading process of future and current sessions.
  • Lazy Load Non-Essential Resources: Delay the loading of non-critical resources such as images and third-party plugins until they are needed.

3. Streamline Server Responses: Reducing server response times can improve how quickly a page becomes interactive.

  • Use Efficient Server-Side Rendering (SSR): SSR can send a fully rendered page to the client, which means the browser spends less time rendering content from scratch.
  • Employ Content Delivery Networks (CDNs): CDNs can reduce latency by storing copies of your content closer to users, speeding up resource delivery.

4. Manage Third-Party Scripts Efficiently: Third-party scripts such as ads, analytics, or social media widgets can degrade FID if not managed properly.

  • Limit Third-Party Impact: Use asynchronous loading for third-party scripts to prevent them from blocking your content. Where possible, defer their loading until after the main content is interactive.
  • Monitor Third-Party Performance: Regularly check the performance impact of third-party scripts and remove any that significantly increase FID.

Advanced Techniques for FID Optimization

Beyond basic optimizations, several advanced techniques can be applied to further reduce First Input Delay (FID) and enhance the user experience on complex websites:

1. Implement Web Workers: Web workers allow you to run JavaScript in the background, on a separate thread from the main browser thread. This means heavy tasks can be processed without interfering with the user interface.

  • Offload Tasks: Identify tasks that do not require direct manipulation of the UI and move them to a web worker. This could include data processing, complex calculations, or logging activities.
  • Responsive Design Practices: Utilize responsive design principles that ensure scripts and features adapt based on the user’s device capabilities, reducing unnecessary processing and improving FID.

2. Optimize Event Listeners: Event listeners can significantly impact FID if not managed correctly, especially if they are extensive and complex.

  • Debounce and Throttle: Implement debounce or throttle techniques to limit the rate at which a function is executed. This is particularly useful for resizing, scrolling, or keypress events that do not need to be detected and acted upon with every single occurrence.
  • Passive Event Listeners: Use passive event listeners to improve scrolling performance on mobile devices. This technique tells the browser that the event listener will not prevent the default action, allowing the browser to continue processing without waiting for the listener.

3. Client-Side Rendering Optimization: For applications heavily reliant on client-side rendering:

  • Selective Hydration: In frameworks like React, selective hydration allows you to prioritize the most critical parts of your application to be interactive sooner. This reduces the time until the user can begin interacting with the most important elements.
  • Incremental Rendering: Techniques like incremental static regeneration (ISR) can be used with Next.js and other frameworks to update static content incrementally rather than reloading the entire page.

4. Prioritize Input Readiness: Ensure that interactive elements are ready to respond as soon as they appear on the screen.

  • Minimize Main Thread Work: Reduce the workload on the main thread during the initial load. This can be done by minimizing the use of synchronous scripts, optimizing CSS delivery, and deferring non-critical tasks.
  • Optimize Critical Rendering Path: Adjust the order and method of loading resources to speed up the time it takes for the most critical parts of your page to become interactive.


Optimizing First Input Delay (FID) significantly enhances user interactions with websites by ensuring responsiveness and usability, directly impacting user satisfaction and engagement. Through diverse strategies such as JavaScript optimization, resource management, and advanced techniques like web workers and selective hydration, businesses can dramatically improve how users experience their sites. Real-world case studies across various industries demonstrate the substantial benefits of focusing on FID, from boosting SEO rankings to enhancing user engagement. Continued success in web performance requires ongoing monitoring, adaptation to emerging technologies, and responsiveness to user feedback, making it crucial for developers to maintain vigilance and innovation in optimizing FID. This proactive approach not only meets but also exceeds modern web user expectations, fostering faster and more efficient web environments.