TTFB (Time to First Byte)
Time from request to the first byte of response. Not a Core Web Vital, but the ceiling on LCP.
Time to First Byte (TTFB) measures how long the server takes to send the first byte of a response. It is not a Core Web Vital, but everything else waits on it, so a slow TTFB caps Largest Contentful Paint no matter how well the front end is optimised. Under 800 milliseconds is the usual target.
Definition
Time to First Byte measures the interval between the browser requesting a page and receiving the first byte of the response. It bundles DNS lookup, TCP and TLS negotiation, the server’s own processing, and network transit.
TTFB is not a Core Web Vital, but it is the floor beneath one: Largest Contentful Paint cannot happen before the first byte arrives, so a slow TTFB caps LCP no matter how well the page is optimised afterwards.
Under 800 ms is the commonly cited target; under 200 ms is achievable with caching and a CDN.
Why It Matters
A high TTFB is usually a server or infrastructure problem, not a front-end one — an uncached database query, a slow origin, a distant server, a redirect chain paid before the real request starts. Optimising images will not move it.
It is also the cheapest metric to fix relative to its effect, because caching a response converts a slow dynamic render into a fast static one without touching the page.
Example
Measure it directly: curl -w "%{time_starttransfer}\n" -o /dev/null -s https://example.com. If the number is high, look at server-side caching, database query time, whether a CDN is serving the HTML, and whether the URL redirects before it responds.
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