Photo Web Performance Audits

Implementing Advanced Web Performance Audits with Lighthouse and Web Vitals Chrome Extension

Tech Guides & Tutorials

You’ve probably heard a lot about making your website faster, and for good reason. A slow website can frustrate users, hurt your search engine rankings, and ultimately, cost you business. The good news is, it’s not some mystical art.

Tools like Lighthouse and the Web Vitals Chrome Extension are designed to help you pinpoint exactly what’s holding your site back and give you actionable steps to fix it.

Think of them as your friendly diagnostic tools, pointing out the squeaks and rattles in your web performance engine.

Before we dive into the how-to, let’s quickly cover what these tools are all about.

What is Lighthouse?

Lighthouse is an open-source, automated tool for improving the quality of web pages. You can run it against any web page, public or requiring authentication. It audits content for:

  • Performance: How quickly your page loads and responds.
  • Accessibility: How easily people with disabilities can use your page.
  • Best Practices: Whether your page adheres to common development best practices.
  • SEO: How well your page is optimized for search engines.

While all these are important, we’re focusing on Performance today. Lighthouse gives you a score from 0 to 100 for each of these categories, and a detailed breakdown of what’s good and what could be better.

What are Web Vitals?

Web Vitals are a set of metrics defined by Google that aim to measure the user experience on the web. They focus on aspects of the user experience: load, interactivity, and visual stability. The three core Web Vitals are:

  • Largest Contentful Paint (LCP): Measures loading performance. It marks the point in the page load timeline when the largest content element in the viewport is likely to have rendered.
  • First Input Delay (FID): Measures interactivity. It quantifies the experience that users feel when trying to interact with your pages. It’s the time from when a user first interacts with your page (e.g. clicks a link, taps on a button) to the time when the browser is actually able to begin processing event handlers in response to that interaction. (Note: FID is being replaced by Interaction to Next Paint (INP) in March 2024, which is a more comprehensive measure of interactivity).
  • Cumulative Layout Shift (CLS): Measures visual stability. It quantifies how often users experience unexpected layout shifts. A layout shift occurs when a visible element changes position from one frame to the next.

The Web Vitals Chrome Extension provides real-time feedback on these metrics as you browse your site, allowing for immediate insight into user experience.

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Key Takeaways

  • Clear communication is essential for effective teamwork
  • Active listening is crucial for understanding team members’ perspectives
  • Setting clear goals and expectations helps to keep the team focused
  • Regular feedback and open communication can help address any issues early on
  • Celebrating achievements and milestones can boost team morale and motivation

Running Your First Lighthouse Audit

Getting started with Lighthouse is pretty straightforward. It’s built directly into Chrome DevTools, making it super accessible.

Accessing Lighthouse in Chrome DevTools

  1. Open your website: Navigate to the page you want to audit in Google Chrome.
  2. Open DevTools: Right-click anywhere on the page and select “Inspect,” or press F12 (or Cmd + Option + I on Mac).
  3. Navigate to the Lighthouse tab: In the DevTools panel, you’ll see several tabs. Click on the “Lighthouse” tab.
  4. Configure your audit:
  • Categories: Make sure “Performance” is checked. You can uncheck others if you’re solely focused on speed.
  • Device: Choose “Mobile” or “Desktop.” It’s crucial to test both, as performance can vary significantly. Mobile is often the more critical scenario.
  • Throttling: This simulates slower network conditions and CPU power. For a true picture of what a user on a less-than-ideal connection experiences, leave this on. “Fast 3G” is a good default.
  1. Click “Analyze”: Lighthouse will then run its checks and present you with a report.

This initial audit is your baseline. Don’t be discouraged if your score isn’t perfect; that’s why we’re doing this.

Interpreting the Lighthouse Performance Report

The report is dense but navigable. Here’s a breakdown of the key sections you’ll want to focus on:

Performance Score and Metrics

At the top, you’ll see your overall Performance score. Below that, you’ll find the core Web Vitals metrics (LCP, FID/INP, CLS) with their current values and how they stack up against Google’s benchmarks (Good, Needs Improvement, Poor).

Opportunities

This is where Lighthouse gives you specific, actionable suggestions to improve your score. Each opportunity typically includes:

  • A recommendation: What you should do.
  • An estimated savings: How much time or how many bytes you could save by implementing the recommendation.
  • A link to documentation: More details on how to implement the fix.

These are often the most valuable part of the report. They’ll point out things like:

  • Eliminate render-blocking resources: Scripts or CSS that prevent the browser from rendering the page until they’re downloaded and processed.
  • Reduce initial server response times: How quickly your server sends the initial HTML document.
  • Properly size images: Serving images that are too large for their display size.
  • Minimize main-thread work: Tasks that the browser’s main thread has to do, which can block user interaction.
  • Reduce JavaScript execution time: How much time JavaScript takes to run.

Diagnostics

This section provides more detailed insights into your page’s performance characteristics. You’ll find information on:

  • Total blocking time: The total time spent by the main thread during page load that was blocked by long-running tasks.
  • First contentful paint: When the first bit of content is rendered on the screen.
  • Time to interactive: When the page is visually rendered and reliably responsive to user input.
  • Server response times (TTFB – Time to First Byte): How long it takes for the browser to receive the first byte of information from the server.

Passed Audits

This is simply a list of checks that your page passed. It’s good to see these, but the focus for improvement will be on the “Opportunities” and “Diagnostics” sections.

Using the Web Vitals Chrome Extension for Real-time Feedback

Web Performance Audits

While Lighthouse gives you a snapshot at a specific point in time, the Web Vitals Chrome Extension offers real-time insights as you interact with your site. This is incredibly useful for debugging and understanding how user actions affect performance.

Installing and Using the Extension

  1. Find the extension: Search for “Web Vitals” in the Chrome Web Store and install it.

  2. Pin the extension: Once installed, click the puzzle piece icon in your Chrome toolbar and then the pin next to Web Vitals. This makes it easily accessible.

  3. Navigate your site: Go to your website or any website.

  4. Click the extension icon: When the extension icon is clicked, it will display the current Web Vitals for the page you’re viewing.

    It updates in real-time as you interact or scroll.

Understanding Real-time Metrics

The extension typically displays:

  • LCP: Shows the Largest Contentful Paint value.

  • CLS: Shows the Cumulative Layout Shift value.

  • FID/INP: Shows the First Input Delay or Interaction to Next Paint value.

This real-time feedback is invaluable because:

  • It helps identify unpredictable issues: Sometimes, performance problems only appear after a certain user interaction or on specific pages that Lighthouse might not have highlighted as prominently.

  • It speeds up the debugging loop: Instead of running a full Lighthouse audit repeatedly, you can make a small change and immediately see its impact on the Web Vitals through the extension.

  • **It helps you understand the impact of your changes:** You can make a code tweak, refresh the page, and instantly see if your LCP improved, if CLS decreased, etc.

The extension is particularly good for understanding how elements on your page are shifting around (CLS) or how quickly content is appearing (LCP).

Deep Diving into Performance Bottlenecks

Photo Web Performance Audits

Lighthouse and the Web Vitals extension are your guides, but you need to know where to look for the most impactful fixes. The “Opportunities” section in Lighthouse is your primary starting point.

Optimizing Images

Images are often the biggest culprits for slow loading times.

Serving Scaled Images

  • Problem: You’re uploading a huge image file (e.g., 4000px wide) but only displaying it at 400px. The browser has to download the massive file and then scale it down, wasting bandwidth and processing power.
  • Solution: Use image editing tools to resize your images to the dimensions they’ll actually be displayed at. Modern image formats like WebP can also offer better compression and quality. Lighthouse will often tell you: “Properly size images.”

Lazy Loading Images

  • Problem: All images on a page load immediately, even those far below the fold (not visible in the initial viewport). This delays the rendering of content that the user can see.
  • Solution: Implement lazy loading. This technique defers the loading of offscreen images until the user scrolls near them. Modern browsers support native lazy loading with the loading="lazy" attribute on tags. Lighthouse will suggest: “Defer offscreen images.”

Next-gen Image Formats

  • Problem: Using older formats like JPEG or PNG when more efficient formats are available.
  • Solution: Convert images to next-gen formats like WebP or AVIF. These formats offer superior compression with little to no loss in visual quality, resulting in significantly smaller file sizes. Lighthouse might suggest: “Serve images in next-gen formats.”

Minimizing and Deferring JavaScript and CSS

Render-blocking resources are a major performance killer.

Eliminating Render-Blocking Resources

  • Problem: Your HTML page can’t start displaying content until all linked CSS and JavaScript files are downloaded, parsed, and executed. This is especially bad for critical CSS and JavaScript needed for the initial above-the-fold content.
  • Solution:
  • CSS: Inline critical CSS (the CSS needed for above-the-fold content) directly into the of your HTML and defer loading the rest of your CSS.
  • JavaScript: Use the defer or async attributes on your