metrics
TTFB (Time to First Byte)
Time from request start to the first byte of response — the floor for every other Core Web Vital.
Definition
Time to First Byte measures the duration from the browser's first request packet to receiving the first response byte. Includes DNS lookup, TCP handshake, TLS negotiation, server processing, and network return trip. A bad TTFB caps LCP from above — you can't have fast LCP with slow TTFB. Good < 800ms; poor > 1.8s.
Example
A WordPress site on shared hosting at 1.4s TTFB. Moving to Vercel Edge Functions at 60ms TTFB unlocks sub-second LCP.How to optimize
Use a CDN with edge POPs near your users. Cache server responses. Use HTTP/3 if available. Pre-render pages or use ISR.