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Performance terms
25 performance terms, defined plainly and cross-referenced.
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CLS (Cumulative Layout Shift)PerformanceCLS measures the largest burst of unexpected layout shifts during the entire lifecycle of a page. Target: under 0.1.Content Delivery Network (CDN)PerformanceA geographically distributed network of proxy servers that deliver content from locations closest to the user, reducing latency and improving load times.Core Web VitalsPerformanceA set of three Google metrics (LCP, INP, CLS) that measure real-world user experience on loading, interactivity, and visual stability.Critical Rendering PathPerformanceThe sequence of steps the browser uses to convert HTML, CSS, and JavaScript into pixels. Minimizing this path is the foundation of frontend performance optimization.CrUX ReportPerformanceThe Chrome User Experience Report is Google's public dataset of real-user performance metrics from opted-in Chrome browsers. It powers field data in PSI and Search Console.
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FCP (First Contentful Paint)PerformanceFCP measures the time from navigation to when the browser renders the first DOM content. Target: under 1.8 seconds.Font Display SwapPerformanceA CSS font-display strategy that renders text immediately in a fallback font while the custom font loads, then swaps to the custom font once downloaded. Prevents Flash of Invisible Text (FOIT).
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Image OptimizationPerformanceThe practice of selecting, compressing, sizing, and delivering images to minimize load time while preserving visual quality. The single highest-impact optimization for most websites.INP (Interaction to Next Paint)PerformanceINP measures page responsiveness by observing the latency of all clicks, taps, and keyboard interactions. Target: under 200 ms.
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LCP (Largest Contentful Paint)PerformanceLCP measures the time from navigation to when the largest visible content element is rendered. Target: under 2.5 seconds.LighthousePerformanceAn open-source Google auditing tool measuring performance, accessibility, SEO, best practices, and PWA readiness. Produces lab-based scores, not field data.
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PageSpeed InsightsPerformanceA Google tool combining Lighthouse lab data with CrUX field data for any public URL. Shows both diagnostic scores and real-user metrics.Partial Prerendering (PPR)PerformanceA rendering strategy pioneered in Next.js that combines static prerendering and dynamic streaming in the same HTTP response. Static shell is served instantly from CDN; dynamic content streams in.Performance BudgetPerformanceA set of agreed-upon limits on performance metrics or resource quantities that a team commits not to exceed. Prevents regressions from shipping.Preload / Prefetch / PreconnectPerformanceResource hints that tell the browser to fetch or connect earlier than it would otherwise. Preload fetches critical resources early. Prefetch fetches resources for future navigation. Preconnect sets up early connections to origins.
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Real User Monitoring (RUM)PerformanceRUM captures performance data from actual users via JavaScript using Performance APIs. You own the data, it covers all browsers, and can capture custom metrics.Render-Blocking ResourcePerformanceA CSS or JavaScript file that the browser must fully download and process before it can render any content to the screen. The most common performance bottleneck on the web.Responsive Images (srcset/sizes)PerformanceAn HTML technique using srcset and sizes attributes to serve different image resolutions based on the user's viewport width and device pixel ratio.
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