Core Web Vitals: First Input Delay – What It Is and How to Improve It for SEO

“Core web vitals” is a set of core webpage functionalities that impact user experience. Google’s ranking algorithm update called page experience, which Google rolled out from June to August 2021, incorporates core web vitals as NEW ranking factors for SEO.

The current set of core web vitals includes:

• Largest Contentful Paint (LCP)
• First Input Delay (FID)
• Cumulative Layout Shift (CLS)

In this article, part of our series covering the page experience update, I’ll discuss first input delay, or FID:

• What is FID?
• How can I measure FID?
• How does FID impact SEO?
• How do I improve my FID score?

What Is First Input Delay (FID)?

FID measures the responsiveness of a page to user events. While technically this could happen throughout the lifespan of a user session on a webpage, in practice, most interactivity problems occur during initial page load. That is because this is when most resources are being downloaded, parsed, executed and rendered.

Google discusses FID here:

First Input Delay (FID) is an important, user-centric metric for measuring load responsiveness because it quantifies the experience users feel when trying to interact with unresponsive pages—a low FID helps ensure that the page is usable. …

FID measures the time from when a user first interacts with a page (i.e., when they click a link, tap on a button, or use a custom, JavaScript-powered control) to the time when the browser is actually able to begin processing event handlers in response to that interaction.

Google cites two important reasons why the first input delay is important:

• The first input delay will be the user’s first impression of your site’s responsiveness, and first impressions are critical in shaping our overall impression of a site’s quality and reliability.
• The biggest interactivity issues we see on the web today occur during page load. Therefore, we believe initially focusing on improving site’s first user interaction will have the greatest impact on improving the overall interactivity of the web.

Keep in mind that FID will not apply to every situation, as Google points out here:

Not all users will interact with your site every time they visit. And not all interactions are relevant to FID … How you track, report on, and analyze FID will probably be quite a bit different from other metrics you may be used to.

How Is First Input Delay (FID) Measured?

FID measures the first impression of your site’s interactivity and responsiveness. It analyzes things like clicks, taps and key presses, which fall under the “responsiveness” category. It does not measure things like scrolling and zooming, which are related to animation.

Google recommends webpages aim for an FID of 100 milliseconds or less*, which means the page would be able to respond to an interactive event within that time frame. In order to be considered “good,” Google’s threshold should be met 75 percent of the time, segmented across mobile and desktop devices. You can learn more about how Google creates thresholds here.
*Updated threshold per Google as of 2/18/2021

Google’s FID score

While the official threshold is 75% of page loads, Google says that for FID in particular, they “strongly recommend looking at the 95th to 99th percentiles, as those will correspond to the particularly bad first experiences users are having with your site. And it will show you the areas that need the most improvement.” This is true for both desktop and mobile users.

For developers, it’s important to understand that Google only measures the delay in event processing, not the “event processing time itself nor the time it takes the browser to update the UI after running event handlers.”

In other words, Google only measures how long the browser takes to start executing the event process. So, if you click on a link, it’s the delay between the time you click and the time the browser starts processing that click.

And when you’re ready to start improving FID, you’ll use tools that can help measure real data in the wild.

How Does First Input Delay (FID) Impact SEO?

Improving FID is another way to speed up your webpages for visitors. Consider that fast page loading was already a best practice for SEO and a ranking factor long before we heard of core web vitals. FID helps keep visitors on your site because they can interact with the content faster.

When people bounce from your site, they may never come back and you can lose potential revenue. Not only that, but a sluggish site can also impact your rankings. That’s because Google’s AI, RankBrain, may take into account how a user engages with the search results.

Over time, if a website has enough visitors who go to the page from the search results and bounce back quickly, this could indicate they didn’t find what they were looking for. Because RankBrain’s goal is to analyze and serve the most relevant search results, rankings could suffer.

The good news is that most sites may already be OK when it comes to FID. In a study by Screaming Frog, 89% of mobile and 99% of desktop URLs fell within the threshold. The average was around 56 milliseconds on mobile and 13 milliseconds on desktop.

When looking at FID and search rankings correlation, Screaming Frog says that there’s much less of a correlation than for other core web vitals. But you need to recall that 2021 is when this becomes an important factor, and we would not expect an impact yet.

“How Many Sites Pass the Core Web Vitals Assessment?,” Screamingfrog.co.uk

How Do I Improve My First Input Delay (FID) Score?

Google provides tools to measure FID, including:

• Chrome User Experience Report
• PageSpeed Insights
• Search Console (Core Web Vitals report)
• Firebase Performance Monitoring (beta)

You can also measure FID with the web-vitals JavaScript library, and you can learn more about that here. If you are serious about improving CWV, this is the best way to get real-time feedback from actual user sessions to determine how to fix FID in the field.

The primary cause of a bad FID score is heavy JavaScript execution. So be sure to optimize how “JavaScript parses, compiles, and executes on your web page will directly reduce FID,” says Google. Reducing the amount of JavaScript and/or optimizing the running of JavaScript has always been a good idea for SEO.

If a user clicks while a JavaScript file is being processed, the browser can’t react and the user feels blocked. If your FID score is in the red, you may need to split up your JavaScript files so the browser can go back and forth between JavaScript processing and reacting to the user.

Optimizing your JavaScript reduces page bloat, improves page performance and also provides Google an easier path to index the correct content. That’s because Google will not have to process as much JavaScript to figure out what it needs.

In our experience, the more you can give Googlebot what it needs right away, without having to process too many things, the better Google will index your site the way you think it should be indexed. Indexing is hugely important for SEO as it influences what pages Google determines are valid or not.

To optimize the FID score, Google recommends running a Lighthouse performance audit and looking at the opportunities uncovered, but gives more detail on how to optimize JavaScript here.

Find out more about the update by reading our page experience series:

1. What’s the Page Experience Update?
2. How to Make a Mobile-Friendly Site
3. Intrusive Interstitials & Why They’re Bad for SEO
4. HTTPS for Users and Ranking
5. Core Web Vitals Overview
6. Core Web Vitals: LCP (Largest Contentful Paint)
7. Core Web Vitals: FID (First Input Delay)
8. Core Web Vitals: CLS (Cumulative Layout Shift)

Watch our on-demand webinar 3 Expert Tips to Improve Core Web Vitals to get more in-depth help on this timely SEO topic.

Bruce Clay is founder and president of Bruce Clay Inc., a global digital marketing firm providing search engine optimization, pay-per-click, social media marketing, SEO-friendly web architecture, and SEO tools and education. Connect with him on LinkedIn or through the BruceClay.com website.

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