TTFB (Time to First Byte)

TL;DR: TTFB is how long the browser waits until the first byte of the response shows up. For a page load it counts redirects, DNS, TCP, TLS and the server’s think time, not only your backend.

Time to First Byte (TTFB) is the time from the start of a request until the first byte of the response is received. In the Navigation Timing API, for a page load it is responseStart - startTime, so everything before the server starts replying is counted: redirects, DNS lookup, TCP and TLS setup, the request itself, and the server’s processing time.

Measuring it

curl -o /dev/null -s -w 'dns=%{time_namelookup} connect=%{time_connect} tls=%{time_appconnect} ttfb=%{time_starttransfer} total=%{time_total}\n' https://example.com/

time_starttransfer is the time to the first byte, measured from the start of the curl run. Subtract time_appconnect to get roughly the server’s wait. In the browser:

const [nav] = performance.getEntriesByType('navigation')
console.log('TTFB ms:', nav.responseStart)

Non-obvious facts

• Two definitions in circulation. DevTools’ “Waiting for server response” is only the wait after the request is sent. Field data (CrUX, RUM, PageSpeed) counts from navigation start. Compare like with like before declaring a regression.
• Redirects count. An http to https hop or a www redirect adds a full round trip, and sometimes a second TLS handshake, before the first byte of the real page.
• Streaming changes what “first byte” means. A server that flushes headers or a shell early gets a low TTFB even if the body takes seconds. Low TTFB alone does not mean the page is fast.
• You can see where the time went. Have the server emit a Server-Timing header (Server-Timing: db;dur=53, app;dur=47) and it appears in DevTools and in the Performance API.
• Common inflators. A cold cache or cache bypass at the CDN, a slow database query, a serverless cold start, a distant origin, and a missing connection reuse to the upstream.

Go deeper

• Server-Timing header
• Cache-Control header
• CDN
• Request and response lifecycle