[AVIF](atomicglue.co/glossary/avif) — AVIF (AV1 Image File Format) is the most compression-efficient image format available, offering 50% smaller files than JPEG and 20-30% smaller than WebP, with HDR and wide color gamut support.
[Browser Caching](atomicglue.co/glossary/browser-caching) — Storing previously downloaded resources on the user's device so subsequent page loads (or return visits) can skip network requests entirely.
[CLS (Cumulative Layout Shift)](atomicglue.co/glossary/cls-cumulative-layout-shift) — CLS measures the largest burst of unexpected layout shifts during the entire lifecycle of a page. Target: under 0.1.
[Content Delivery Network (CDN)](atomicglue.co/glossary/content-delivery-network-cdn) — A geographically distributed network of proxy servers that deliver content from locations closest to the user, reducing latency and improving load times.
[Core Web Vitals](atomicglue.co/glossary/core-web-vitals) — A set of three Google metrics (LCP, INP, CLS) that measure real-world user experience on loading, interactivity, and visual stability.
[Critical Rendering Path](atomicglue.co/glossary/critical-rendering-path) — The 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 Report](atomicglue.co/glossary/crux-report) — The 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.
[Edge Caching](atomicglue.co/glossary/edge-caching) — Storing pre-generated responses on CDN edge servers so subsequent requests are served without contacting the origin server. Critical for low TTFB.
[FCP (First Contentful Paint)](atomicglue.co/glossary/fcp-first-contentful-paint) — FCP measures the time from navigation to when the browser renders the first DOM content. Target: under 1.8 seconds.
[Font Display Swap](atomicglue.co/glossary/font-display-swap) — A 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).
[Gzip / Brotli Compression](atomicglue.co/glossary/gzip-brotli-compression) — Server-side compression algorithms that reduce the size of HTML, CSS, and JavaScript files before sending them to the browser. Brotli typically achieves 20-30% better compression than Gzip.
[Image Optimization](atomicglue.co/glossary/image-optimization) — The 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)](atomicglue.co/glossary/inp-interaction-to-next-paint) — INP measures page responsiveness by observing the latency of all clicks, taps, and keyboard interactions. Target: under 200 ms.
[LCP (Largest Contentful Paint)](atomicglue.co/glossary/lcp-largest-contentful-paint) — LCP measures the time from navigation to when the largest visible content element is rendered. Target: under 2.5 seconds.
[Lighthouse](atomicglue.co/glossary/lighthouse) — An open-source Google auditing tool measuring performance, accessibility, SEO, best practices, and PWA readiness. Produces lab-based scores, not field data.
[PageSpeed Insights](atomicglue.co/glossary/pagespeed-insights) — A 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)](atomicglue.co/glossary/partial-prerendering-ppr) — A 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 Budget](atomicglue.co/glossary/performance-budget) — A set of agreed-upon limits on performance metrics or resource quantities that a team commits not to exceed. Prevents regressions from shipping.
[Preload / Prefetch / Preconnect](atomicglue.co/glossary/preload-prefetch-preconnect) — Resource 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.
[Real User Monitoring (RUM)](atomicglue.co/glossary/real-user-monitoring-rum) — RUM 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 Resource](atomicglue.co/glossary/render-blocking-resource) — A 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)](atomicglue.co/glossary/responsive-images-srcset-sizes) — An HTML technique using srcset and sizes attributes to serve different image resolutions based on the user's viewport width and device pixel ratio.
[Streaming SSR](atomicglue.co/glossary/streaming-ssr) — A server-side rendering technique that sends HTML to the browser in chunks as they become available, rather than waiting for the full page to render. Dramatically improves FCP and Time to First Byte.
[TTFB (Time to First Byte)](atomicglue.co/glossary/ttfb-time-to-first-byte) — TTFB measures the time from navigation request until the first byte of the server response arrives. Target: under 800 ms.
[WebP](atomicglue.co/glossary/webp) — A modern image format from Google offering 25-35% smaller files than JPEG at equivalent quality, with support for transparency and animation.