Definition
TTFB is not a Core Web Vitals metric but is a core input to LCP. It has five components: redirect, DNS lookup, connection setup, TLS negotiation, and request waiting.
How it works
The browser starts the request. DNS resolution, TCP handshake, and TLS negotiation complete. The server processes the request and sends the first byte. Each component is measured. CDN edge proximity, server-side cache, database query time, and middleware chain determine TTFB. Under 200 ms is good; above 500 ms warrants review.
Where you see it in Scope Trends
Site Crawl > Performance shows TTFB components per page. CDN and cache hit ratio appear in the Server Health report.
Frequently asked questions
TTFB thresholds?
Good <= 800 ms, needs improvement 800-1800 ms, poor > 1800 ms.
Does a CDN always reduce TTFB?
For static content, yes. For dynamic HTML, only with edge cache or stale-while-revalidate.