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First Input Delay (FID) Explained

· 5 min read
Oliver Whitfield
Oscar Weissnat

First Input Delay

First Input Delay (FID) is a Core Web Vital that measures the Interactivity of a website.

Important

First Input Delay will get replaced by Interaction to Next Paint (INP) as a Core Web Vital in March 2024.

What is First Input Delay (FID)?​

First Input Delay (FID) is a crucial metric in understanding the performance of a website. It measures the time between a user's first interaction with a webpage and the moment the browser begins processing that interaction.

This metric is part of Google's Core Web Vitals and is used as a minor ranking factor, making it an essential aspect of website optimization.

FID is about first impressions. It gauges the user's initial experience with your page, which is fundamental in their perception of your web performance.

Most of the blocking of the browser's main thread occurs in the first moments of a webpage's lifecycle when critical resources are loaded. FID helps address this issue, ensuring that loading these resources doesn't make your website feel unresponsive.

However, it's important to note that FID only measures the delay in input, not the processing time.

It doesn't account for the time it takes for the webpage to update as a result of the interaction. This is because developers could manipulate FID by separating the event handler from the task associated with the event.

How to Measure FID​

FID can be measured using Lab Tools, Field Tools, JavaScript, and also using the Web Vitals Chrome Extension.

Lab Tools​

Lab tools are used to simulate user interactions in a controlled environment. They provide a consistent way to measure FID across different code changes. However, they can't perfectly mimic real-world conditions and user behavior. Tools like Lighthouse and WebPageTest are examples of lab tools.

Field Tools​

Field tools, on the other hand, capture real-world user experience data. They provide insights into how actual users are experiencing your site.

Examples of field tools include Google's PageSpeed Insights, the Chrome User Experience Report (CrUX), and Search Console's Core Web Vitals report.

Measuring in JavaScript​

JavaScript can also be used to measure FID. The PerformanceEventTiming interface provides data that can be used to calculate FID. This interface captures the duration between the user's first interaction and the browser's response. However, it's important to note that this method requires adding additional JavaScript to your page, which could potentially impact performance.

What is a Good FID Score​

FID Score Range

A good FID score is less than 100 milliseconds. This ensures that your site is responsive and provides a smooth user experience.

An average FID score ranges between 100 to 300 milliseconds, and anything above 300 milliseconds is considered poor.

High FID scores can lead to a frustrating user experience, potentially causing users to abandon your site.

How To Improve FID​

Improving FID involves optimizing your site's JavaScript and how it's loaded. Large JavaScript files can block the main thread, leading to long input delays.

Minimizing or deferring non-critical JavaScript, breaking up long tasks, and using a web worker can help reduce the impact of JavaScript on FID.

Optimizing your CSS can also improve FID. Avoiding large, complex layouts and layout shifts can help reduce the time it takes for the browser to render content, improving both FID and the overall user experience.

  1. Minimize or Defer JavaScript: Large JavaScript files can block the main thread, leading to long input delays. You can minimize the impact by reducing the size of your JavaScript files or deferring the loading of non-critical JavaScript until after the first paint.

  2. Break Up Long Tasks: Long tasks, defined as any task that takes the browser more than 50 milliseconds to complete, can delay the processing of user inputs. Breaking up these tasks into smaller, manageable chunks can help improve FID.

  3. Use a Web Worker: Web workers allow you to run JavaScript in the background, separate from the main execution thread. This can help prevent large JavaScript files from blocking the main thread and delaying user input.

  4. Optimize Your CSS: Avoiding large, complex layouts and layout shifts can help reduce the time it takes for the browser to render content, improving both FID and the overall user experience.

  5. Implement Lazy Loading: Lazy loading is a technique where you defer loading of off-screen content until the user needs it. This can significantly reduce the amount of work the browser needs to do when the page first loads, improving FID.

  6. Use Browser Caching: Leveraging the browser cache can help improve FID by storing static files, like CSS and JavaScript, in the user's browser. This means the browser can load these files from the cache rather than downloading them again, reducing the time it takes to load the page.

By implementing these strategies, you can improve your FID score, leading to a smoother, more responsive user experience.

Conclusion​

First Input Delay is a vital metric for understanding user experience on your website. It provides insights into how quickly your site responds to user interactions, which can directly impact user engagement and satisfaction.

By optimizing your JavaScript, you can improve your FID score, leading to a smoother, more responsive user experience.