Definitions of key web performance terms and metrics
Largest Contentful Paint
LCP measures the time it takes for the largest content element (image, video, or text block) to become visible within the viewport. It's a key indicator of perceived loading speed. Good LCP is under 2.5 seconds.
Interaction to Next Paint
INP measures the latency of all user interactions throughout the page lifecycle, reporting the longest interaction. It replaced FID as a Core Web Vital in March 2024. Good INP is under 200 milliseconds.
Cumulative Layout Shift
CLS measures the total of all unexpected layout shifts that occur during the entire lifespan of the page. A layout shift happens when a visible element changes its position. Good CLS is under 0.1.
First Contentful Paint (FCP)
FCP measures the time from when the page starts loading to when any part of the page's content is rendered on screen. This marks the first point where the user sees something happening.
Total Blocking Time (TBT)
TBT measures the total amount of time between FCP and TTI where the main thread was blocked long enough to prevent input responsiveness. It's the lab equivalent of INP.
Speed Index (SI)
Speed Index measures how quickly content is visually displayed during page load. It captures how fast the page appears to fill in with content from the user's perspective.
Time to First Byte (TTFB)
TTFB measures the time it takes for the browser to receive the first byte of response from the server. It reflects server response time and network latency.
Time to Interactive (TTI)
TTI measures the time it takes for the page to become fully interactive – meaning the page displays useful content, event handlers are registered, and the page responds to interactions within 50ms.
Real-world performance data collected from actual Chrome users visiting your site over the past 28 days. This data comes from the Chrome User Experience Report (CrUX).
Performance data collected in a controlled environment using Lighthouse. Lab data is collected under fixed conditions and is useful for debugging specific performance issues.
Google's open-source, automated tool for improving the quality of web pages. It audits performance, accessibility, SEO, and more. It powers PageSpeed Insights.
A public dataset of real user experience data on millions of websites. It provides the Field Data shown in PageSpeed Insights and is the source of Core Web Vitals data for Google Search ranking.
Resources (typically CSS and JavaScript) that prevent the browser from rendering page content until they are downloaded and processed. Reducing render-blocking resources improves LCP and FCP.
