Performance & CWV
17 terms in this domain · show all 220
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Core Web Vitals (CWV)
Core Web Vitals (CWV) are three field-measured metrics — Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS) — that Google uses as signals of real user experience. A page passes CWV when all three are in the "good" band at the 75th percentile.
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CrUX Report (CrUX)
The Chrome User Experience Report (CrUX) is Google's public dataset of real-user performance metrics, collected from opt-in Chrome users on public websites. Powers the field data in PageSpeed Insights, the Core Web Vitals report in Search Console, and the basis for ranking signals. 28-day rolling window.
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Cumulative Layout Shift (CLS)
Cumulative Layout Shift (CLS) measures the sum of unexpected layout shifts during a page visit. Each shift is scored by impact fraction × distance fraction. A page passes CLS when the total is ≤ 0.1 at the 75th percentile of real-user visits.
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Interaction to Next Paint (INP)
Interaction to Next Paint (INP) measures the worst interaction latency during a page visit — time from a user's tap, click, or keypress until the browser paints the next frame. Replaced FID as a Core Web Vital in March 2024. Good: ≤ 200 ms at the 75th percentile.
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Intrusive interstitial
An intrusive interstitial is a modal that blocks the primary content of a mobile page right after the user arrives from search. Google has used it as a ranking signal since January 2017. Cookie banners, age verification, and login walls for paywalled content are explicitly exempt from the penalty.
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Lab vs Field Data
Lab data is synthetic — a single page load under controlled, throttled conditions, repeatable run-to-run. Field data is real users — millions of loads under varied devices, networks, and locations. Google ranks on field data (CrUX); you optimize against lab (Lighthouse) because it's diagnostic and reproducible.
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Largest Contentful Paint (LCP)
Largest Contentful Paint (LCP) is the time from navigation start until the largest image or text block in the initial viewport renders. It's one of Google's three Core Web Vitals. Good: ≤ 2.5 seconds at the 75th percentile of real-user visits.
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Lighthouse
Lighthouse is Google's open-source auditing tool for web pages. It runs five category audits — Performance, Accessibility, Best Practices, SEO, and PWA — and produces scores from 0-100 plus actionable diagnostics. Generates lab data, not field data. Available in Chrome DevTools, CLI, Node, and PageSpeed Insights.
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Real User Monitoring (RUM)
Real User Monitoring (RUM) is the practice of measuring performance and Core Web Vitals from actual user sessions rather than synthetic lab tests. CrUX is the public RUM dataset Google uses for ranking; private RUM tools (SpeedCurve, DebugBear, Datadog, New Relic) add per-session granularity, custom dimensions, and authenticated-page coverage.
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Render-blocking Resources
Render-blocking resources are CSS or JavaScript files in the document `<head>` that the browser must download and process before it can render anything. They delay First Contentful Paint and Largest Contentful Paint. Common fixes: inline critical CSS, async/defer JS, split CSS by media, preload the LCP element.
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Resource Hints
Resource hints are HTML link directives that tell the browser to set up connections or fetch resources earlier than the parser would discover them. The five: dns-prefetch, preconnect, preload, prefetch, and modulepreload. Each targets a different stage of the loading pipeline. Used carelessly, they hurt — used surgically, they cut LCP.
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Time to First Byte (TTFB)
Time to First Byte (TTFB) is the interval between the browser sending a request and receiving the first byte of the response. It captures DNS, TCP, TLS, request queueing, and server processing. Google's thresholds: good under 800ms, needs improvement 800-1800ms, poor above 1800ms. Strong upstream correlate of LCP.
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Time to Interactive (TTI)
Time to Interactive (TTI) is the lab metric for when a page becomes reliably responsive — first FCP, then 5 seconds of network and main-thread quiet. Removed from Lighthouse's scoring weight in v10 (2023) and effectively deprecated as a public metric. INP replaced it for measuring real-user interactivity.
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Total Blocking Time (TBT)
Total Blocking Time (TBT) is the sum, in milliseconds, of the portions of long tasks (>50ms) between FCP and TTI that exceed 50ms. Lab-only metric in Lighthouse. The closest synthetic proxy for INP, the field Core Web Vital for interactivity. Good under 200ms, poor above 600ms.