Time to First Byte (TTFB) measures how long after a request the server starts returning data. Under 800ms is considered good, 800ms to 2.5s is acceptable, over 2.5s is poor. TTFB is foundational to Core Web Vitals because it caps LCP, and matters for AEO because aggressive AI crawlers time out on slow servers before they finish parsing the page.
Server response time (database queries, application logic, rendering). Network latency between user and server. DNS lookup time. SSL/TLS handshake. CDN hit rate (cache hits return in milliseconds; cache misses go all the way to origin).
Use a CDN (Cloudflare, Fastly, Bunny). Cache HTML responses. Optimize database queries (add indexes, denormalize hot reads, use a read replica). Choose a host with low latency to your audience. Move to HTTP/3 if available.
Lighthouse reports TTFB. Chrome DevTools Network tab shows it on every request. The free Readiness Check measures TTFB on every audited URL.
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