Explore Our Core Web Vitals Articles

Learn how to test and continuously monitor Core Web Vitals for any page using Testdom.io. This step-by-step guide walks you through running your first test, enabling automated monitoring, customizing device and network settings, and understanding detailed performance reports. You’ll also discover how to view Lighthouse scores, interpret metrics like LCP and TTFB, and uncover optimization opportunities through visual loading timelines, waterfall charts, and issue breakdowns. Whether you’re a developer or site owner, this guide helps you stay in control of your website’s speed and user experience.

Largest Contentful Paint (LCP) is a Core Web Vitals metric that measures how long it takes for the largest visible content element — typically an image or block of text — to fully render on screen. A fast LCP improves perceived load time, user satisfaction, and SEO performance. Common causes of poor LCP include slow server response, render-blocking resources, unoptimized images, and client-side rendering delays. To optimize LCP, you can use techniques such as server-side rendering, lazy loading, image compression, caching, CDNs, and resource preloading. Monitoring LCP helps ensure your pages load quickly and deliver a smooth user experience.

The Search Index (SI) metric evaluates how well a web page is prepared for indexing and visibility in search engines. Unlike visual performance metrics, SI focuses on SEO-readiness — including metadata presence, crawlability, structured data, and canonical tags. A high SI score means your page is discoverable, indexable, and optimized for search ranking. Pages with low SI often lack critical elements like titles, descriptions, or proper robots directives. Improving SI involves adding structured data, internal links, meta tags, and fixing noindex issues. Regularly monitoring SI helps ensure your content is not just fast, but also findable and SEO-compliant.

Time To First Byte (TTFB) is a web performance metric that measures how quickly a user’s browser receives the first byte of data from the server after making an HTTP request. A fast TTFB indicates a responsive server and optimized backend, which directly impacts user experience and SEO rankings. High latency during this phase can cause delays in page rendering and hurt Core Web Vitals scores. Optimizing TTFB involves reducing server processing time, leveraging caching strategies, using Content Delivery Networks (CDNs), and minimizing database or application logic delays. Monitoring TTFB helps diagnose and fix server-side performance bottlenecks effectively.

Time To Interactive (TTI) is a performance metric that measures how long it takes for a web page to become fully interactive — meaning it responds quickly to user input without lag. A fast TTI is critical for user experience, especially on mobile, where delays can lead to frustration and abandonment. Slow TTI is often caused by heavy JavaScript execution, long tasks blocking the main thread, or third-party scripts. Optimizing TTI involves code splitting, deferring non-critical JS, minimizing main thread work, and using performance tools like Lighthouse. Monitoring TTI ensures your pages are not just visible but also usable.

Unused JavaScript increases load time and wastes bandwidth. This guide explains what causes this issue in Lighthouse reports, how it affects performance, and practical steps to eliminate it using tree-shaking, lazy-loading, and proper server configurations.

Core Web Vitals are essential performance metrics that influence both user experience and search engine rankings. They measure loading speed, interactivity, and visual stability — key factors in how users perceive your site. In this article, you’ll learn what each metric means, why it matters, and how to optimize it. We’ll also explore powerful tools like Lighthouse, PageSpeed Insights, and how Testdom.io automates tracking and sends alerts when something goes wrong — so your site stays fast, stable, and SEO-friendly at all times.

First Contentful Paint (FCP) is a performance metric that measures how long it takes for the browser to render the first visible content — such as text, images, or SVGs — after the user navigates to a page. It’s a key signal of perceived load speed and directly influences user trust and engagement. A fast FCP reassures users that the site is loading, even if it’s not yet interactive. Delays in FCP are often caused by slow server response, render-blocking CSS or JavaScript, and unoptimized resources. Optimizing FCP improves both user experience and Core Web Vitals performance.

Cumulative Layout Shift (CLS) measures how much visible content unexpectedly moves around while a page loads. Sudden layout shifts — like buttons jumping or text shifting — frustrate users and lead to accidental clicks, hurting the overall experience. A good CLS score means your site feels stable and polished, boosting user satisfaction and SEO rankings.

Understanding and improving CLS is crucial for optimizing your website’s performance. In our article, we break down what causes layout shifts, how to measure them, and practical techniques to keep your pages steady. Whether you’re a developer or site owner, mastering CLS helps you create smoother, more reliable user experiences. Dive into the full guide to learn more!