Google’s Web Vitals: first input delay (FID). How long do I have to wait to interact with the site?

What it is and how to optimize the metric which measures user interactivity? What are at stake here are search engine rankings and conversions, primarily from mobile devices.

Have you ever forgotten what you were searching for on a site because the response time to your click on a button or a link was longer and more tedious than you had expected? This happens more often than you might think, especially on mobile devices. The potential loyal customer then abandons that site with a “This is the last you’ll be seeing of me” mindset.

This subjective experience as confirmed in a Chromium study, suggests that a user’s attention span on a mobile device does not exceed 8 seconds, and recommends that the optimum time for an expected response is 4 seconds. One of the various objectives set by the Google Page Experience, the update aimed at improving UX online, is also the following: preventing disastrous or limiting awkward interaction as much as possible between users and a site, especially from smartphones, where searches and conversions are increasingly more concentrated.

Not surprisingly, First Input Delay is one of the new Core Web Vitals, a trio of user-centric metrics introduced by Google in May 2021 to measure and quantify page experience.  In fact, FID measures site responsiveness when a user first interacts with a site. This metric is likely to become one of the more critical ones, especially for mobile versions, heavily impacting site rankings in search results.  As a matter of fact, Google considers Core Web Vitals as ranking factors in the SERPs and so ignoring them can have dire consequences for organic traffic and conversions.

But they also present an opportunity: implementing the changes suggested in the update and improving those parameters means improving your own visitors’ and clients’ user experience regardless of ranking in search engine results. Various surveys have highlighted how a delay of just one tenth of a second can affect up to 10% of conversions while one extra second can lose up to 15%. To make it clear, here is some data provided by Google support regarding website performance:

  • Loading times between 1-3 seconds, bounce rate frequency +32%
  • Loading times between 1-6 seconds, bounce rate frequency+106%

Measuring interactivity: what it is and how First Input Delay is calculated

Before delving into what exactly First Input Delay measures and how to improve this important metric, let’s quickly run through the three Core Web Vitals:

Largest Contentful Paint (LCP): it is the loading speed factor and measures the time taken to render the biggest content, image or text block on the visible area of the page.

First Input Delay: it is the interactivity factor and measures the time it takes for the user’s browser to respond when a user first interacts with a site.

Cumulative Layout Shifts (CLS):  it measures visible stability by summing all individual visible layout shifts while a site is loading.

So, FID evaluates the quality of the user experience in terms of page speed interactions.  It should be made clear that, like the other new signals, this metric is based on the user’s real-world experience during site navigation and not on a lab-controlled setting. 

To the point, First Input Delay calculates the time lapse between a user’s first interaction with a page, which could be a click on a link or a button, and the browser’s actual first response to that interaction in milliseconds.   Crucially, it measures the first input delay in event processing and not the event processing time as a whole, which, if split up into individual responses, would be useless for improving the metric. Zooming and scrolling are more related to other parameters and so are not included in this measurement.

As can be seen from the table, a delay which is equal to or less than a 100 milliseconds, one tenth of a second, is classified as good in relation to Google Page Experience parameters, whereas anything up to 300 milliseconds is tolerable but in need of improvementAnything above this threshold is classed as poor from a good user experience perspective. The same Google suggests that

“to ensure you’re hitting this target for most of your users, a good threshold to measure is the 75th percentile of page loads, segmented across mobile and desktop devices.”

But beware: although Google sets all Core Web Vitals thresholds at the 75th percentile, it would be more useful to set the threshold for FID at the 95th-99th percentiles given the variability of this parameter from user to user. An upward shift allows a webmaster to identify the worst user experiences and flags up the elements that are possibly in need of improvement in order to better interactivity on the web site pages.

What delays interaction and how to fix it

It must be remembered that FID, a parameter which reflects the real user experience when interacting with the site, can be measured using various tools

But what can delay the start of an interaction? 

 A delay generally occurs when the browser’s main thread is already busy processing something else, especially as is the case in large JavaScript file which, when running, takes priority and can block the main thread thus leading to an unresponsive page. The browser then has to wait until the task completes before it can respond to the input.

In short, if a site is likely to make a first bad impression on a user due to frustratingly long waiting times while looking for articles, or worse still, products and services, the main culprit is usually heavy JavaScript execution.  Having to intervene with programing language or resort to workarounds so as to lighten the load of the main thread and reduce delay times from the user’s first interaction, is fatal.

The requested actions are not always simple, fast and painless and good results cannot be guaranteed.  Possible actions are as follows:

  •  shortening JS working time 
  • minimizing third- party code
  • splitting up main tasks into many smaller tasks consecutively.
  • compressing JavaScript code and removing unused code
  • reducing the number of requests
  • lowering transfer size

The solution

So does that mean that it’s all wrong, and needs to be done again? In theory yes, but there is a quick and easy fix: the installation of a smart layer that puts everything back in order to allow Google to find the website semantically and figure in the rankings, while leaving the functionality and basic architecture of the site unaltered.

This is precisely what Aritmetika’s iSmart Frame does, which – without affecting the configuration of the site, which would risk compromising its delicate ecosystem – transforms in Javascript into HTML and renders it, without touching the animation Java. In other words, it makes a site readable by Google without affecting the framework. Not only that, but iSmartFrame considerably improves the website user experience, which is essential for conversion, by improving another of the parameters that Google prizes: loading speed. A double positive benefit, which is entirely independent of the original complexity of the website, increasing both functionality and efficiency.


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