Demystifying Google’s Core Web Vitals: First Input Delay

In the second instalment of our SEO: Demystifying Google’s Core Web Vitals series – where we dissect each of the core web vital metrics in an easy and practical manner to prepare you for what lies ahead – we’ll be discussing First Input Delay (FID), the measurement of time between an interaction and a response.

Joe Comotto

What is First Input Delay (FID)?

First Input Delay (FID) is the second metric of the three Core Web Vitals being introduced by Google. Each of the three metrics focus on different elements of user experience, with FID being the metric used by Google to measure the interactivity of a page. 

“The lights are on but nobody’s home”

Google explains FID as the measurement of time from when a user first interacts with a page to the time when the browser is actually able to respond to that interaction. To put it simply, FID is the delay between when you interact (ie. click or tap) with something like a link or a button, and the time that the browser responds to your action and starts processing it. 

Why is First Input Delay Important for SEO?

It’s important because Google has decided to incorporate it into its algorithm from May, so a poor FID could cause a negative impact to your website as you appear lower down on the search engine results page. 

FID is a measure of how quickly a user can engage with your site and Google wants to make sure that, along with returning the most relevant search results, it’s returning the sites that offer users the best experience as well. FID provides a user-centric metric for measuring load responsiveness – quantifying the experience users feel when trying to interact with unresponsive pages.

How do you measure First Input Delay?

FID is measured in milliseconds and, to receive the highest perceived responsiveness of the page, you want to keep this as low as possible, 

Google classes a good FID scoring as under 100 milliseconds, with a score between 100 and 300 milliseconds requiring improvement. If your FID exceeds 300 milliseconds then this is considered to be poor, and your page experience score, and credibility is jeopardised. If you’re seeing a significant number of your pages fall into this category then you can expect that visitors to your site are receiving a very poor experience.

It’s worth noting that FID measurement can only be calculated using field data and can’t be derived from lab data. This is because it requires visitors to interact with your site and can’t be modeled, much like the other metrics can.

How to Improve your FID

The most common cause of a poor FID is usually down to JavaScript, and render blocking resources. Removing 3rd party code, keeping the number of request counts low and reducing JavaScript execution time will all help improve FID.

You can do this by:

  • Breaking up long tasks
  • Merging and minifying
  • Deferring unused JavaScript
  • Using web workers.

There’s also a strong relationship between the time it takes from First Contentful Paint (FCP) to Time to Interactive (TTI), and First Input Delay.

The time between FCP & TTI is the time that it takes the browser to render the page fully. During that time, if a user makes an interaction with a web page, there will most likely be a delay due to network requests blocking the main thread. Advertisers should consider focusing on refining their page rendering if the time between first contentful paint and time to interactive is over a second or so.

Want to learn more? Explore the previous article in the series Demystifying Google’s Core Web Vitals – Largest Contentful Paint (LCP). Or more information on what the algorithm update will look like, have a read of our previous article Google’s New Search Criteria to familiarize yourself with Page Experience Signals, and what the update will mean for the marketing industry.

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