Optimizing First Input Delay

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December 24, 2022

Max Barinov

The Web: a vast digital landscape where speed is paramount. You're likely well aware that web performance is a key factor in user experience and SEO rankings. At the heart of this performance revolution are the Core Web Vitals, a set of metrics Google uses to quantify the user experience's quality. Today, we're going to zoom in on one of these crucial metrics: First Input Delay (FID).

Understanding First Input Delay

Before diving into optimization strategies, let's establish a fundamental understanding of First Input Delay. As defined by Google, FID measures the time from when a user first interacts with your site (clicks a button, taps a link, etc.) to the time the browser responds to that interaction 1.

In simple terms, FID is the time it takes for your website to "react" when a user "asks" it to do something. A shorter FID translates into a more responsive, and thus more user-friendly, website. Google considers an FID of 100 milliseconds or less as good, whereas an FID above 300 milliseconds is poor.

The beauty of FID as a metric is that it doesn't just measure loading speed. It quantifies the actual user experience, reflecting the delay users perceive when trying to interact with your site.

However, not all actions a user might take on your site contribute to the FID. Only discrete events such as clicks, taps, and key presses that are blocked by the main thread are considered. Continuous events, like scrolling and zooming, do not affect FID.

Why First Input Delay Matters

FID is essential for several reasons. First, it's a critical component of Google's Core Web Vitals, which significantly influence a site's ranking in search engine results. A website with a low FID has a better chance of ranking higher in Google's search results, enhancing its visibility and click-through rate.

Second, FID directly impacts user experience. A website that responds promptly to user interaction provides a more positive user experience, increasing engagement and potentially improving conversion rates. It's no secret that users are likely to abandon a website if it doesn't react quickly to their inputs 2.

The Technicalities Behind First Input Delay

Here's where we get to the techy stuff. The primary cause of a long FID is JavaScript.

When the browser's main thread is busy parsing and executing a large JavaScript file, it cannot respond to user interactions. This period of unresponsiveness is what contributes to a high FID.

Typically, the main thread is blocked by:

  • Long tasks: A task that runs for more than 50ms. Long tasks can be due to JavaScript parsing, layout, rendering, or garbage collection.
  • JavaScript execution: When the browser parses, compiles, and executes JS.
  • Render-blocking resources: Resources that prevent the site from displaying its content until they're fully downloaded and processed.

Let's explore these areas in more detail and identify some strategies for optimization.

Optimizing JavaScript Execution

One of the most effective ways to reduce FID is by optimizing your JavaScript (JS). Here are a few strategies:

  1. Minimize JS: Minifying JS involves removing unnecessary characters (like spaces and comments) without changing functionality. This reduces the file size, leading to faster download times and less blocking of the main thread. Tools like UglifyJS and Terser can help with this.

  2. Compress JS: Compression can also significantly reduce file size. Gzip and Brotli are popular compression methods, and most servers and CDNs can be configured to use them.

  3. Remove Unused JS: Removing unused JS can make a significant impact on FID. Tools like PurgeCSS, UnCSS, or Chrome DevTools Coverage can help identify and eliminate unused JS.

  4. Use Code Splitting: By splitting your JS into smaller chunks, only the necessary code is loaded for each page. This is particularly useful for single-page applications (SPAs) that often load the entire JS file upfront 3.

  5. Implement Lazy Loading: Lazy loading delays the loading of non-critical resources until they are needed. This technique can significantly reduce the initial payload and speed up page load times.

Taming Long Tasks

Long tasks are JavaScript tasks that take more than 50ms to complete. These tasks block the main thread, increasing the FID. Here are a few strategies to tame these tasks:

  1. Break Up Long Tasks: If you have tasks that take more than 50ms to complete, try breaking them up into smaller, asynchronous tasks. This can be accomplished using techniques like requestIdleCallback() or requestAnimationFrame() 4.

  2. Use Web Workers: Web Workers run scripts in the background, off the main thread. This can prevent long tasks from blocking user interactions.

  3. Limit Third-Party Usage: Third-party scripts, particularly those that load synchronously, can significantly contribute to FID. Limit their usage or load them asynchronously when possible.

Dealing with Render-Blocking Resources

Render-blocking resources, including some types of JavaScript and CSS, can significantly impact FID as they must be loaded and processed before the page can render 5. Here's how to deal with them:

  1. Inline Critical CSS: By inlining critical CSS (the minimum CSS required to render the above-the-fold content), you can eliminate the need for an extra network request.

  2. Defer Non-Critical CSS: For non-critical CSS, you can use media queries or the preload attribute to defer their loading until after the critical rendering path.

  3. Defer Non-Critical JS: Using the defer attribute in your script tags, you can instruct the browser to execute the scripts after the document has been parsed.

  4. Async Download of Non-Critical Resources: For non-critical resources, use the async attribute to download them in the background while the rest of the page continues to parse.

Server-Side Rendering (SSR) and Static Site Generation (SSG)

SSR and SSG are two techniques that can significantly improve the FID.

In SSR, the server generates the HTML for each request, which can reduce the amount of JS the browser needs to download, parse, and execute. Meanwhile, SSG generates the HTML at build time, making it an excellent choice for sites with content that doesn't change frequently.

Frameworks like Next.js and Gatsby.js provide robust support for SSR and SSG 6.

Measuring First Input Delay

To make effective improvements, we need a way to measure FID. As FID is a field metric, it can't be simulated in a lab environment 7. Instead, it requires real user data to measure accurately. Several tools can help with this, including:

  1. Google's PageSpeed Insights and Lighthouse: These tools don't measure FID directly, but they measure Total Blocking Time (TBT), a lab metric that correlates strongly with FID. A high TBT likely means a high FID. Lighthouse is integrated into Chrome DevTools, making it easily accessible for developers.

  2. Chrome User Experience Report (CrUX): CrUX is a public dataset of real user experience data on millions of websites. It measures FID along with other Core Web Vitals.

  3. Google Search Console (GSC): GSC provides a Core Web Vitals report where you can check how your site performs on real-world FID.

  4. Web Vitals JavaScript library: This JS library developed by Google provides a way to measure Core Web Vitals metrics, including FID, in the field.

  5. Real User Monitoring tools like Loado: These tools provide insights using dashboards, helping developers to improve user conversions and optimize SEO strategies.

In Summary

First Input Delay is a crucial metric for understanding user experience and plays a significant role in Google's Core Web Vitals and SEO rankings. By optimizing JavaScript execution, taming long tasks, dealing with render-blocking resources, and leveraging techniques like SSR and SSG, you can significantly improve your site's FID.

Remember, the key to optimizing FID, as with any performance metric, is continuous monitoring and iterative improvement. By staying vigilant about performance and utilizing the measurement tools available, you can provide a user experience that both your visitors and Google's algorithms will appreciate.

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