Web performance is crucial for a seamless user experience, and Core Web Vitals helps assess just that. They focus on load performance, interactivity, and visual stability. Let’s break down four essential metrics:
Largest Contentful Paint (LCP)
Cumulative Layout Shift (CLS)
Interaction to Next Paint (INP)
Time to First Byte (TTFB).
Largest Contentful Paint (LCP)
Definition: LCP measures the time taken to render the largest visible element within the viewport, such as an image, video, or large text block. It’s an indicator of how quickly the main content becomes visible to users.
Why It’s Important: LCP reflects the user’s perception of how fast the main content loads. An LCP below 2.5 seconds is considered good, as it makes users feel the page loads quickly and efficiently.
Common Causes of Poor LCP:
Slow server response times
Render-blocking resources like CSS and JavaScript
Large image files that aren’t optimized
Client-side rendering delays
Improvement Tips:
Optimize images by using next-gen formats like WebP
Minimize CSS and JavaScript blocking
Leverage caching and Content Delivery Networks (CDNs) to reduce load times
Cumulative Layout Shift (CLS)
Definition: CLS measures the visual stability of a web page by tracking unexpected shifts of content on the screen as the page loads.
Why It’s Important: Unstable layouts can lead to a frustrating experience, especially when users try to click a button or link only to have it move unexpectedly. A CLS score below 0.1 is ideal, as it indicates minimal visual shifting.
Common Causes of Poor CLS:
Loading images or ads without dimensions
Inserting content above existing content on the page
Dynamic elements without reserved space, like pop-ups or banners
Improvement Tips:
Specify width and height attributes for images and videos
Reserve space for dynamic content and ads to prevent unexpected shifts
Use CSS to manage animations and transitions to avoid abrupt changes
Interaction to Next Paint (INP)
Definition: INP measures the latency of the most important interaction on a page, capturing how responsive the page is when a user interacts with it. This is calculated from the latency of all interactions on a page, focusing on those that matter most to the user.
Why It’s Important: High INP scores indicate responsiveness issues that can cause frustration for users. An INP below 200 milliseconds is considered optimal, as it ensures interactions feel immediate and fluid.
Common Causes of Poor INP:
Heavy JavaScript execution causing delays
Slow response times from event listeners
Large DOM sizes that slow down rendering
Improvement Tips:
Minimize JavaScript execution time and avoid excessive scripting
Use web workers to offload tasks
Reduce DOM size and simplify layouts to improve response times
Time to First Byte (TTFB)
Definition: TTFB measures the time taken for the browser to receive the first byte of data from the server after a user makes a request.
Why It’s Important: A quick TTFB indicates that the server is responding quickly, while a slow TTFB can signal server or network delays. A TTFB below 200 milliseconds is ideal, as it reflects a responsive server and network.
Common Causes of Poor TTFB:
Slow server performance
Network latency and distance between server and user
Non-optimized server configurations or high-traffic loads
Improvement Tips:
Use CDNs to serve content from locations closer to the user
Optimize server configurations and consider caching dynamic content
Minimize redirects, which add extra round trips to the server
Understanding "This URL" and "Origin" in PSI Reports
In PageSpeed Insights (PSI), performance scores are broken down to give a detailed view of how well a website performs both for a specific page (URL) and across the entire domain (Origin). Here’s what each term means and how it relates to the Core Web Vitals metrics—LCP, CLS, INP, and TTFB.
This URL:
Definition: “This URL” displays performance data specifically for the individual page you’re testing. It includes metrics such as LCP, CLS, INP, and TTFB, showing how this specific page performs for users.
Purpose: This section is particularly useful when analyzing a critical page—like a homepage, product page, or checkout page—to identify if any page-specific elements are causing slowdowns. By focusing on individual pages, you can catch specific issues (like large images or heavy scripts) that may not affect other pages.
Note: If the Field data for your current URL(This URL) is missing, the Origin is being used instead.Practical Use: Use “This URL” insights when troubleshooting specific pages or optimizing high-traffic areas of your site. For example, you might notice that an unoptimized hero image on a product page is impacting the LCP, prompting you to compress it for faster load times.
Origin:
Definition: “Origin” provides an aggregated view of Core Web Vitals metrics across all pages within the same domain (e.g., all pages on yoursite.com). This section helps you understand overall performance trends and issues at the domain level, capturing broader performance patterns across multiple pages.
Purpose: The “Origin” section is essential for assessing your site’s overall health. It can reveal if there are systemic issues affecting multiple pages, such as server response times (impacting TTFB) or site-wide layout shifts (impacting CLS).
Practical Use: Use “Origin” insights to find out if there are broader, site-wide optimizations needed. For instance, if you see a high TTFB across multiple pages, this could indicate the need for a CDN or server optimization to reduce server response times across the site.
Summary
Each of these metrics—LCP, CLS, INP, and TTFB—provides critical insights into different aspects of web performance, directly impacting the user experience. Optimizing these can lead to faster load times, smoother interactions, and a more stable, visually pleasing interface.