What is First Input Delay? A Complete Guide

What is First Input Delay? A Complete Guide

December 18, 2023

If you own a website, what would you prefer? People coming to your website and becoming loyal customers? Or, leave and never come back? Of course, the first! But, for that, you need to give a wonderful experience to your users when they visit your website. After all, the first impression is everything! Now, a first impression can be different, depending on the preference of the user. Some prefer design and visual appeal, while some prefer speed and responsiveness. You can measure the speed of a website through the First Contentful Paint (FCP) Score. Similarly, to measure the responsiveness of a website, you need to check the score of First Input Delay (FID).

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What is the First Input Delay (FID)?

First Input Delay (FID) is a performance metric that measures the time between a user first interacting (clicking, or tapping) with a webpage and how long it takes the browser to process the interaction. For example, if you click on the “Register Now” button, does it instantly redirect you to a new page, or do you have to wait for a moment before the browser responds to the click?

The time between these two actions is known as Input Delay (aka Input Latency). This could happen when the primary thread of a browser is busy with a different request.

Why Does Input Delay or Input Latency Happen?

In the case of web development, input delay or input latency can happen when the main thread of the browser is occupied with other tasks such as parsing or executing heavy JavaScript files. This prevents the browser to respond immediately to a user request.

Such large JavaScript codes can make your website feel responsive. It feels like your browser is stuck in traffic, and waiting for the road to clear up before it moves forward.

Are First Input Delay and Time to Interactive the same?

TTI checks how much time it takes for a page to be fully interactive after the initial page load, while FID checks the input latency before the page becomes fully interactive.

To be more specific, TTI checks when the main content is loaded, event handlers are registered and the page becomes responsive to user input. TTI gives information on how quickly users can engage and interact with the content on the page.

FID primarily focuses on the responsiveness of a page when a user inputs something as it checks the perceived responsiveness of a page. That means, a low FID score equals to a more responsive and interactive user experience.

Time to Interactive (TTI) is calculated at a point when the whole content is rendered on a page, event handlers are registered for most page elements, and user interaction is processed within 50ms. You can get the score of Time to Interaction from Lighthouse.

FID tracks the score when a user clicks on a link before event handlers are registered for most page elements. That means, FID allows you to capture the early interactions.

First Input Delay (FID) and its importance for SEO

As we have discussed earlier, the FID score is based purely on real user metrics. That means the results and assessment are more accurate and critical for a website. It cannot be calculated through lab simulations, as it only registers the experiences of real users when they enter a page.

First Input Delay is now officially a ranking factor for Google.</a  FID tracks the website’s responsiveness, CLS tracks visual stability, and LCP assesses the loading speed of a page. Together, these three become Core Web Vitals, Which are an essential part of search engine optimization (SEO).

What is an ideal FID score?

FID is measured in milliseconds (ms), so the lesser the score, the better the responsiveness.

According to PageSpeed Insights, an FID score of 100ms or less is considered good. Meanwhile, FID between 100-300ms requires some improvement, while a score above 300ms is poor.

How to Measure the First Input Delay of a Webpage?

First Input Delay (FID) can be measured through tools like:

Chrome User Experience Report (CrUX):

It is a publicly available dataset to check real user performance data, including the FID metric. You can access the CrUX dataset through tools such as Google Data Studio, PageSpeed Insights, and Looker Studio.

Search Console Core Web Vitals Report

Log in to your Google Search Console and go to the “Core Web Vitals” report. The report displays the metric type (CLS, FID, LCP and INP) grouped by performance (Good, Needs Improvement and Poor).

Web Vitals JavaScript library

You can use the web-vitals library to measure the web vitals metric or FID of your webpage.

Here’s how you can use the library to measure FID:

import { getFID } from &#39;https://unpkg.com/web-vitals@3/dist/web-vitals.iife.js&#39;;
// Measure FID and log the result
// Import the necessary function to measure FID
import { getFID } from &#39;https://unpkg.com/web-vitals?module&#39;;

// Measure FID and log the result

The getFID functions measure the First Input Delay of a webpage, and you can also log the results for future analysis. Moreover, you can integrate the web-vitals library with Google Analytics as well.

First Input Delay (FID) measure web vitals javascript Library

PageSpeed Insights
This tool from Google also includes FID measurements for your web pages.

Enter the website or webpage link, and click on “analyze” The report will provide information about several metrics including FID. You will also get suggestions on how to improve the score.

Pagespeed insights measurement, web vitals score checking

Lighthouse in DevTools

You can use Lighthouse in DevTools to check FID score, through running a Lighthouse Audit in Chrome DevTools.

Right-click on a webpage and select “inspect” to open the DevTools panel.

Now, click on the Lighthouse tab (appear as two arrows at the end of the toolbar).

Select the five categories and choose “Generate Report

The Total Blocking Time (TBT) feature in Lighthouse can be used to measure FID.

How to Optimize First Input Delay (FID) of your Webpage?

A lot of the optimization revolves around manually optimizing JS code. You may need the help of experienced web developers in specific cases. However, the performance gains will undoubtedly help you increase your user experience and improve SEO scores.

Optimize and Compress CSS and JavaScript Codes

Remove unnecessary line breaks, characters, and spaces, ensuring you have a smaller file size. A simple way to do that is to use the Code Minifier tool or get it done by a web developer.

A minified code will reduce the number of the process a browser has to go through. As a result, it increases the speed and responsiveness of a page.

Split Long JavaScript tasks

Long JavaScript tasks block the main thread of a page, not allowing the browser to process user input. As a result, it lowers your site’s responsiveness.

The best way to overcome these issues is to break down these tasks into smaller tasks. Make sure your tasks remain under 50ms.

The ideal way to split codes is by breaking them up and loading smaller pieces of code at a time, instead of using a large file. You can check out the GeekonGeek Guidelines for code splitting.

Reduce third-party codes and non-critical scripts

One essential step to optimize FID is through reducing the impact of third-party code. Also, remove all non-critical third-party scripts and defer the codes that cannot be removed.

To check the importance of each script, you must carefully analyse which scripts to load first and which ones to delay. For starters, you can remove or delay unnecessary ads or pop-ups. You can check your PageSpeed insights reports under the Opportunities section for recommendations. It contains all the files and tasks that you can potentially block your main thread.

Use async or defer for JavaScript to be executed only as needed.

Use Idle until urgent

It is a great way to keep your input delay to a minimum. The Idle Until Urgent allows code execution during idle periods, thereby, minimizing main thread blocking time. It runs code during idle periods, allowing the main thread to load completely. However, it also loads important code immediately.

The idle until Urgent method uses two code evaluation methods: Eager Evaluation and Lazy Evaluation.

Eager Evaluation allows you to run all the code instantly. As a result, your page loads for a long time until it becomes fully interactive. Lazy Evaluation runs code only when it is needed.

Use Web Workers

Web Workers allows you to run scripts in the background without blocking the main thread. As a developer, you can use them to move non-UI operations like data processing to a separate thread, ensuring a responsive user interface.


First Input Delay (FID) can vary significantly based on when a user initially interacts with a webpage. This variability arises because the browser’s main thread isn’t consistently busy throughout the entire lifecycle of the page. Consequently, some users may encounter no delay when interacting, while others might experience a noticeable delay. Hope, this guide helps you understand more about First Input Display and its role in SEO and provides a seamless user experience.

For more information or to optimize your website’s FID score and core web vitals, you can contact our web development team. Dzinepixel is a leading digital marketing company in Bhubaneswar, India, offering onshore and offshore services in web development, web designing, digital marketing, SEO services, content marketing and writing, social media marketing, paid marketing services and many more.