Time to First Byte (TTFB) is the elapsed time between a user's HTTP request and the first byte of the server's response. It measures the speed of server-side processing, DNS resolution, TLS handshake, and network round-trip combined — the work that happens before the browser sees any HTML.
The 2026 thresholds are under 800ms for "good" per Google's web.dev guidance, under 200ms for "excellent." TTFB feeds into Largest Contentful Paint (you cannot render LCP until you have HTML to parse) but is also weighed as its own signal by AI-engine crawlers.
For AEO, TTFB is critical because PerplexityBot and OAI-SearchBot have tighter response-time budgets than Bingbot or Googlebot. A page with 3-second TTFB gets sampled less frequently by AI crawlers, dropped from the index faster under rate budgeting, and treated as less authoritative even when content is strong. Optimizing TTFB is one of the highest-ROI single technical-SEO investments for AI visibility.
The fastest path to a low TTFB is edge caching for public pages, eliminating synchronous database calls in the request path, using a global CDN, and serving static or stale-while-revalidate content where possible. Server-rendered React pages with no per-request data are typically under 100ms TTFB on a modern edge runtime.