The ultimate guide to web performance

The ultimate guide to web performancePlay button
Video Duration
~7 mins

Mastering Web Performance: A Comprehensive Guide to Optimizing Page Load Speed


In the digital age, capturing the attention of users within the first few seconds is crucial. With the plethora of options available for instant gratification, web developers must prioritize initial page load performance. A slow website can lead to loss of potential customers and revenue. However, optimizing performance is a complex task. This guide aims to demystify key concepts such as largest contentful paint, first input delay, and cumulative layout shift. It also introduces tools like Unlight House and the new Web Vitals Chrome extension to measure and optimize these metrics. By the end of this guide, readers will not only understand these concepts but also how to apply them to their projects.

Key Takeaways

  • Understanding and optimizing web performance metrics is crucial for user engagement and retention.
  • Largest contentful paint, first input delay, and cumulative layout shift are key metrics to measure web performance.
  • Tools like Unlight House and the new Web Vitals Chrome extension can help measure and optimize these metrics.
  • Real-world analysis of popular websites can provide insights into web performance optimization.

Table of Contents

Understanding Web Performance Metrics

When a website loads, it fetches various assets like CSS, HTML, JavaScript, images, and videos. The first contentful paint is the initial loading of these assets, while the largest contentful paint (LCP) is the point at which all images and videos have loaded. Google recommends achieving LCP within 2.5 seconds. Anything beyond four seconds can negatively impact search engine optimization.

Optimizing Largest Contentful Paint

Optimizing LCP involves reducing resource load time, serving assets from a content delivery network, and avoiding render-blocking JavaScript. Images should be compressed and modern formats like WebP should be used. Fonts should be minimized to only what is necessary to render the page. Render-blocking JavaScript, which delays LCP, should be avoided. This is why server-side frameworks like NextJS are favored over single-page applications like plain React.

Optimizing First Input Delay

First input delay (FID) measures the time from when a user interacts with the page to when the browser processes event handlers for that interaction. A good FID is 100 milliseconds or less. To optimize FID, reduce JavaScript execution time. This can be achieved by using tools like web workers and party town to move some JavaScript into a worker, or by using lazy loading options in JavaScript frameworks.

Optimizing Cumulative Layout Shift

Cumulative layout shift (CLS) measures visual stability. Elements on the page should not jump around unexpectedly. To optimize CLS, images should always have a width and height, or use the CSS aspect ratio property. For responsive images that need to change their aspect ratio based on the viewport, use source set to use different images under different conditions.

Tools for Measuring and Optimizing Web Performance

The Chrome team at Google has released a new Web Vitals extension that provides console logs for CLS and LCP. It identifies the elements affecting these metrics, making it easier to pinpoint what needs to be optimized. Another tool, Unlight House, runs a Google Lighthouse report on every page of a website, identifying pages with poor performance.

Real-World Analysis of Web Performance

Analysis of popular websites like Amazon, Google, and Reddit shows that most major websites perform well, with an average performance of around 90. The Astro framework homepage stands out with near-perfect scores across the board. Using tools like Unlight House and the Web Vitals extension can provide valuable insights into web performance optimization.


Understanding and optimizing web performance metrics is crucial in today's digital landscape. By using the right tools and strategies, web developers can significantly improve page load speed, user engagement, and ultimately, the success of their websites.

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