Time to First Byte (TTFB)
How long it takes the browser to receive the first byte of HTML from the server — a server-side performance signal upstream of LCP.
In long form.
TTFB is the foundation of front-end performance — every other timing metric is downstream of it. It's the sum of network round-trip, server processing, and any redirects. Good TTFB is under 200ms (excellent), 200-500ms (acceptable), over 500ms (problematic). Causes of slow TTFB: under-provisioned servers, expensive database queries, missing caching layer, geographically-distant origin, redirect chains. CDN edge caching and ISR (in Next.js) can collapse TTFB from seconds to milliseconds.
When LCP is slow, we always check TTFB first. A 2.5-second LCP with a 1.5-second TTFB means the front-end is fine — the server is the problem. Fixing TTFB is a server-side conversation, not a CSS conversation.
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