Time to First Byte (TTFB)
TTFB is the time from the start of navigation until the first byte of the HTML response arrives. Every other loading metric sits on top of it.
What’s inside TTFB
Field TTFB (as reported in the Chrome UX Report) includes everything before the response starts: redirects, service-worker startup, DNS lookup, TCP and TLS connection, and the server’s time to generate the response. Lighthouse’s lab “server response time” audit is narrower: it only counts the root document request, and flags anything over 600 ms.
How to check your TTFB
Run the CoreVitals checker: the field section shows real-user TTFB at p75, and the lab section shows Lighthouse’s server response time for a single request. For a quick command-line reading, curl -o /dev/null -s -w '%{time_starttransfer}\n' https://example.com prints seconds to first byte from your location. For detail, the Network panel in Chrome DevTools splits each request into DNS, connection, “waiting for server response” and download.
How to reduce TTFB
- Cache HTML at the edge. A CDN serving cached pages from a nearby location is the biggest single win; use
stale-while-revalidatefor pages that change. - Kill redirect chains.
http → https → www → /en/can add a full second on mobile. Link to the final URL. - Speed up the backend. Profile slow database queries, add object caching, and avoid calling external APIs before sending HTML.
- Stream the response. Send the
<head>early (early flush or 103 Early Hints) so the browser can fetch CSS and the LCP image while the server finishes. - Use modern protocols. HTTP/2 or HTTP/3 and TLS 1.3 shave connection round trips.
Check it on your own page
Field and lab numbers for mobile and desktop, in about 30 seconds.
Questions people ask
What is a good TTFB?
Google’s guidance for field data is 0.8 seconds or less at the 75th percentile. Lighthouse flags lab server response times above 600 ms.
Is TTFB a Core Web Vital?
No. It is a diagnostic metric. But a slow TTFB makes a good LCP almost impossible, because LCP can never be earlier than TTFB.
Why is my TTFB slow only for some visitors?
Distance from your server, slow mobile networks, redirects from ad links and cache misses all vary per visitor. A CDN narrows that spread.