Speed metrics and Core Web Vitals
What LCP, INP, CLS, TTFB and the other speed numbers mean, which marks count as good, and how to read your own scores.
Speed is not one number. Google's Core Web Vitals measure three things a visitor feels: how fast the main content shows up, how fast the page answers a tap, and whether the layout stays still while it loads. Lighthouse adds a few more numbers that help you find the cause.
Start here. Read Core Web Vitals if the terms are new, or keep the Core Web Vitals cheat sheet open. Then read the guide for the metric your report flags: LCP, INP or CLS. The supporting numbers are TTFB, FCP, Total Blocking Time and Speed Index.
To fix a failing metric, go to how to fix a slow LCP, how to fix INP or how to fix CLS, or take the whole job in order with how to improve the Core Web Vitals.
To understand your scores, read why lab data and field data disagree, how the Lighthouse score is calculated and why scores change. For a sense of where the web stands, see the 2025 benchmarks.
Guides in this topic 19 guides
- Core Web Vitals in 2026: LCP, INP and CLS explained
The three Core Web Vitals measure loading, responsiveness and visual stability. Google's good marks, how a page passes, and where to start fixing one.
- What is LCP? How to improve Largest Contentful Paint
LCP times how long the biggest item in view takes to appear. Google's good mark is 2.5 seconds or less. The four parts of LCP and how to fix each one.
- What is INP? Interaction to Next Paint explained
INP replaced FID as a Core Web Vital on March 12, 2024. What it measures, Google's 200 ms mark, and how to find and fix pages that react slowly.
- What is CLS? How to fix Cumulative Layout Shift
CLS scores how much a page jumps around while it loads and is used. Google's good mark is 0.1 or less. How the score works, the usual causes and the fixes.
- What is TTFB? How to lower Time to First Byte
TTFB is how long you wait for the first byte of a page. Google's good mark is 0.8 seconds or less. What it includes, what slows it, and how to cut it.
- What is FCP? How to improve First Contentful Paint
First Contentful Paint is when a page first shows text or an image. Good is 1.8 seconds or less. What delays it and how to fix each cause.
- Total Blocking Time (TBT): what it is, how to lower it
Total Blocking Time adds up how long JavaScript kept a page unresponsive while it loaded. Good is under 200 ms on mobile. What causes it and how to cut it.
- What is Speed Index? How Lighthouse scores it
Speed Index measures how quickly a page fills in visually while it loads. Lighthouse scores it, but it is a lab number and not a Core Web Vital.
- How the Lighthouse performance score is calculated
The 0 to 100 Lighthouse score is a weighted average of five metrics. The weights, the scoring curves, worked examples and why 90 is not 100.
- Lab data vs field data: why your speed scores differ
A lab test and real-visitor data measure different things. What each shows, why the numbers disagree, and which one to trust for which decision.
- The Chrome UX Report (CrUX) explained
CrUX is Google's dataset of real Chrome visits, behind PageSpeed Insights and Search Console. Who is in it, why a page may show no data, and how to read it.
- Does page speed affect Google rankings?
What Google says about speed and ranking, what it does not say, and how to treat speed: a real but modest factor next to relevance and content.
- How to fix a slow LCP, step by step
A practical recipe for Largest Contentful Paint over 2.5 seconds: find the element, split the time into four parts, and fix the longest one first.
- How to fix Cumulative Layout Shift, step by step
A practical recipe for a CLS above 0.1: tell load shifts from later ones, find what moved, and fix images, embeds, fonts, banners and late content.
- How to fix a slow INP, step by step
A practical recipe for an Interaction to Next Paint over 200 ms: find the slow interaction, read its three parts, and fix the part that is longest.
- How to improve Core Web Vitals: a practical checklist
A checklist for passing LCP, INP and CLS: which fix helps which number, the order to work in, and a four-week plan for a typical business site.
- Why your PageSpeed score changes between tests
Run PageSpeed Insights twice and get two scores. The real causes of the swings, which changes are noise, and how to test so you can trust the number.
- Page weight and requests: do they matter?
Total page size and the number of files both affect speed, but not equally. What a typical page weighs, what Lighthouse flags, and what to cut first.
- How does your site compare? Speed benchmarks from 2025
Where the web stands on Core Web Vitals, loading, interactivity and layout stability, per the 2025 Web Almanac, and how to use the numbers wisely.
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