CWV – What is First Input Delay (FID)?

First Delay Input (FID) is one of Google's Core Web Vitals (CWV). FID is an important user-centric metric for measuring load responsiveness, according to Web.dev. It helps quantify a user’s perception of a responsive or unresponsive page. 

How is FID measured?

The FID metric helps measure a user's first impression of your site's interactivity and responsiveness. It begins when a user first interacts with a page (for example, when they click a link, scroll down, tap a slider) to the time when the browser begins processing event handlers. 

It is measured in milliseconds.

What is a good FID score?

Like most user-centric Core Web Vitals, a good FID indicates a high-quality user experience. Google wants your FID to be less than 100 milliseconds.

  • Great = less than 100 milliseconds 
  • Needs improvement = between 100 milliseconds and 300 milliseconds
  • Poor = more than 300 milliseconds 

Publishers should strive for the fastest loading page speed possible, which actively contributes to higher organic Google rankings and strong SEO. As Core Web Vitals like FID are incorporated into Google’s algorithm, users will begin to expect these rapidly loading pages. 

What contributes to a poor FID score?

FID only measures the delay in event processing, meaning it does not measure the event processing time itself nor the time to update the user interface. It takes a real user interacting with the page, and FID cannot be tested in a lab.

The most common causes of a poor FID are:

  • Long tasks where a browser must pause
  • Lengthy Javascript execution
  • Heavy Javascript bundles
  • Render-blocking Javascript

Anything that creates unnecessary times between input and your page’s response contributes to a low FID.

Long tasks where a browser must pause

A slow response time means it takes too long for the browser to receive content from the server, creating a slow page load experience for the user. A faster server improves the load speed of your entire site and contributes to a good FID. 

Lengthy Javascript execution

Scripts and stylesheets are both rendered blocking assets that delay a page load, consequently negatively affecting your FID and Google ranking. Optimize your page for interaction readiness by moving costly scripts off the critical path. 

Heavy Javascript bundles

While the main thread is executing Javascript, a browser cannot respond to user interactions, creating the delay that causes the user to tap their mobile screen multiple times or click on a website again and again. Action you can take to address this:

  • Optimize and compress large images
  • Preload important resources
  • Compress text
  • Cache your assets

Render-blocking Javascript

Third parties like analytics tracking platforms or ad tech vendors can unintentionally increase page latency. Ensure third parties aren’t contributing to keeping your network busy and periodically creating unresponsive times.

How can I measure FID?

FID is different from the other two Page Experience CWVs, LCP and CLS. As First Input Delay measures the actual user’s experience and interaction, FID requires a user to engage with the page. 

In addition to tracking your pages' FID scores over time in Sortable’s analyzable, you can measure your FID score in more detail using:

Assess your current Core Vitals by heading into your Google Search Console to examine each web property. 

How can I improve FID on my website?

Once you’ve pulled your report, it’s time to improve your FID. 

Overall, there are four (4) simple ways to improve FID on your website.

Minimize and defer Javascript

Only render the minimal code needed and remove any heavy styling. This speeds up how fast the browser can start responding to user interactions. 

Divide long tasks

Long tasks are defined as a JavaScript execution that blocks the main thread for 50 milliseconds or more. When trying to reduce Javascript lag, you can break down large Javascript bundles into smaller, asynchronous tasks. Long tasks mean your user is waiting on page loads, unable to do anything, and splitting up long tasks reduces input delay on your site. 

Other actions you can take to address long tasks:

  • Spread out code loading and features
  • Consider shifting more logic server-side or generating static content
  • Minimize your reliance on cascading data fetches 

Use a web worker

Web workers make it possible to run JavaScript on a background thread, and a blocked main thread is one of the biggest contributors to input delay. Moving non-critical operations to a separate worker thread frees up the main thread time to improve FID. 

Implement a browser cache

When looking to optimize FID, it’s critical to use caching to help the browser keep local copies of static assets to reduce page load time and minimize overall network traffic. It takes longer to fetch assets from a network versus storing them on a local cache. 

While a user will fetch everything from the server on their first visit, once the user has visited the site before, the browser will retrieve the HTML page on the web server but grab local assets for static properties like JavaScript, CSS, images, etc.

Remove non-critical third-party scripts

Especially when working with multiple ad tech vendors, ensure they are using asynchronous technology and not dragging down page speed, costing you not only revenue but high organic Google rankings. 

Perform a regular cleanup and optimization of non-critical third-party scripts.

How does FID affect my website SEO?

Google values fast sites, and page speed is already a well-known attribute in its ranking algorithm. Slow sites lead to high bounce rates and user frustration, which will inevitably bring your site down in Google rankings.

For publishers looking to diversify traffic acquisition, organic Google search is an important channel to consistently optimize.

A poor FID will negatively impact your search engine optimization:

  • Lower organic search traffic 
  • Difficulty ranking on page 1 for desired keywords and terms 
  • Decreased ad revenue due to fewer impressions, page views, and clicks 
  • High bounce rates 

Google Core Web Vitals and SEO go hand in hand, and it’s important to pay attention to the organic Google ranking effects.

Where can I find more information?

For more information to help improve your FID score, visit the web.dev site.

To further understand Core Web Vitals and what they mean for publishers’ Google rankings, read more about other Core Web Vitals:

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