Render-blocking resource
PerformanceDefined term
A render-blocking resource is one the HTML parser or renderer must wait for: stylesheets referenced in the head and classic synchronous scripts without defer or async. Until they arrive and execute, the page cannot produce its first meaningful paint.
The mitigation toolkit is standardized: defer and async attributes, media-conditional stylesheets, preload for late-discovered critical assets, and inlining truly critical CSS. Every render-blocking byte sits directly on the Largest Contentful Paint path.
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Browse the glossary
Performance7 of 28 terms
- Largest Contentful Paint (LCP)⦿ liveThe render time of the largest visible content element — the loading-speed Core Web Vital. Good ≤ 2.5 s at p75.Performance
- Interaction to Next Paint (INP)⦿ liveThe responsiveness Core Web Vital: full interaction latency through the next visual update. Replaced FID in 2024. Good ≤ 200 ms.Performance
- Cumulative Layout Shift (CLS)⦿ liveThe visual-stability Core Web Vital: the largest session window of unexpected layout shifts. Good ≤ 0.1.Performance
- Time to First Byte (TTFB)⦿ liveElapsed time from navigation start until the first byte of the response arrives — the server and network's share of loading delay.Performance
- Render-blocking resourceA stylesheet or synchronous script the browser must fetch and process before it can paint — a direct LCP tax.Performance
- stale-while-revalidateA Cache-Control extension (RFC 5861) that serves an expired cached response instantly while refreshing it in the background.Performance
- 75th percentile (p75)⦿ liveThe assessment point for Core Web Vitals field data: the experience at least three quarters of page loads meet or beat.Performance