Tarun Gupta

What is First Input Delay (FID) and How to Optimize It?

Tarun Gupta | Jan 4th, 2023 | Digital Marketing

First Input Delay (FID) is an important Core Web Vitals metric to measure web performance. The metric calculates the delay between the user’s first interaction with your page and the moment when the browser starts processing that particular interaction. This is a crucial metric to measure a website’s loading speed which is now an important ranking factor.

What is First Input Delay or FID?

The FID is calculated as the delay between when a user clicks on a link or a button and the time that the browser takes to respond to this action and starts processing it further. The metric is measured in milliseconds. The limitation of this metric is that it can only measure discrete interactions and not continuous user interactions like zooming or scrolling the page.

One of the most important units of Core Web Vitals, FID measures the first impression on the website. The first impression occurs when a user first interacts with a web page. Most of the events related to the browser’s main thread occur in the first moments of a page’s life cycle.

First Input Delay helps website administrators to ensure that loading the resources during the first interaction doesn't impact its performance. The FID however doesn't measure the consequences of the first interaction that occurred. This is because FID is about input delay, not processing.

How First Input Delay is different from Time to Interactive?

Time to Interactive (TTI) is a metric that measures the time it takes for a page to be fully interactive. On the contrary, First Input Delay measures user input before the page is fully interactive. First Input Delay captures early critical interactions, whereas, Time to Interactive is displayed in Lighthouse.

Why it’s important to improve the First Input Delay (FID)?

First Input Delay measures the actual interaction. It’s all about measuring the actual experience of a user when he lands on a web page. Since it’s used to define a website’s user experience, it needs to be monitored and optimized well.

In addition to First Input Delay, Google’s core web vitals include two other major performance metrics including Cumulative Layout Shift and Largest Contentful Paint. All three are the core ingredients of Google’s ranking algorithm that the company rolled out as a Page Experience update in June 2021.

Among the three, First Input Delay tracks a website’s responsiveness, CLS tracks visual stability, and LCP measures loading speed.

How to improve the First Input Delay?

Delay or remove non-critical third-party scripts:

Monitoring third-party scripts can help you improve FID since such scripts might prevent your own scripts from executing properly. So it’s important to prioritize scripts and remove non-critical third-party scripts.

Utilize Web Workers:

Web workers help to improve the first input delay to an extent. Using the web workers, you can run scripts in the background without affecting the Main Thread. It’s always a good option to move non-UI operations to a background thread to boost the website's performance.

Perform minification and compression

Minification and compression can impact performance since they are used to remove unnecessary parts such as comments, whitespace, and line breaks from your code. With Compression, you can rewrite the binary code within a file. Both Minification and compression reduce the file size and make code clean and easier to execute.

Reduce unused CSS

Apart from Javascript, unused CSS in native codes can also affect and block the main thread. You can reduce the bytes that are downloaded during the execution by reducing unused CSS. Besides optimizing JS code, you can take the help of a proprietary resource loader to rearrange the way resources.

Downsides of First Input Delay (FID)

Even though the FID is an important metric in terms of site performance, Google doesn't think so. The tech conglomerate is reportedly working on a better responsiveness metric. FID is a metric that doesn’t track the full event duration. Instead, it measures delay only.

The problem with the FID is that to measure the interaction in real time, it needs user input at a specific time. Without input, it’s difficult to measure anything.

The upcoming metric will calculate the whole event from the moment a user first interacts with the site. Another downside with the metric is that it only tracks the delay after the first input. It doesn’t elaborate on the user experience.


First Input Delay (FID) measures the delay between a user’s first interaction and the moment the browser can respond. Since the metric needs real user input, it’s hard to measure. It’s an important ranking signal and directly impacts the user experience. 

Tagged In: Google

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