Time to first byte (TTFB)
Time to first byte (TTFB) measures the interval between the browser starting a navigation request and receiving the first byte of the server's response. The Performance Timeline derives it from responseStart minus the request's start time, so it folds in redirect, DNS, connection, TLS, and server processing time. Because nothing can render before bytes arrive, a slow TTFB delays every downstream metric, which is why web.dev treats it as a diagnostic for First Contentful Paint and Largest Contentful Paint.
What this means
TTFB is the delay from the moment a request starts to the moment the first byte of the response is received. The Performance Timeline computes it as responseStart minus startTime (or fetchStart) on the navigation entry, so it captures the whole front-of-response journey, not just server compute.
What it includes
Because it is anchored to the request start, TTFB sums up redirect time, DNS lookup, TCP connection, TLS negotiation, and the server's own processing and queueing. web.dev recommends reading those sub-phases of the navigation timing to find which one dominates before optimising.
- TTFB = responseStart − startTime on the navigation entry
- Includes redirects, DNS, connect, TLS, and server time
- Gates FCP and LCP — nothing paints before bytes arrive
Why it misleads
A single TTFB number hides which phase is slow, and field TTFB varies with the visitor's network and distance to the server. Cached or CDN-served responses can show a very low TTFB that does not reflect origin performance, so read it as a distribution across real users rather than one lab figure.
How it appears in analytics and logs
A high TTFB means the response was slow to begin, before any rendering — it points at redirects, DNS, connection setup, TLS, or back-end processing rather than at front-end JavaScript or layout.
Diagnostic use case
Use TTFB to separate server and network delay from front-end rendering cost when a page feels slow, since a high TTFB caps how fast paint metrics can ever be.
What WebmasterID can help detect
WebmasterID can record performance timing events first-party, so you can watch TTFB alongside human-classified traffic and keep bot requests out of the timing distribution.
Common mistakes
- Reading one lab TTFB as representative of all users' networks.
- Blaming the server when redirects or DNS dominate the time.
- Ignoring TTFB when chasing paint metrics it already caps.
Privacy and accuracy notes
TTFB is a timing measurement of the connection, not the person. It needs no identifiers and is educational, not legal advice on data handling.
Related pages
- First Contentful Paint (FCP)
First Contentful Paint (FCP) measures the time from navigation start to when the browser first renders any DOM content — text, an image, a non-white canvas, or SVG. The Paint Timing API exposes it as the first-contentful-paint entry, and web.dev treats it as the moment a visitor first sees that something is happening. It precedes Largest Contentful Paint, which marks the largest element, so the two answer different questions about perceived load.
- Largest Contentful Paint (LCP)
Largest Contentful Paint (LCP) reports the render time of the largest image or text block visible in the viewport, measured from when the page starts loading. It is one of Google's Core Web Vitals, exposed through the Largest Contentful Paint API, and the candidate element can change as larger content paints — the final value is taken at the last candidate before user interaction. web.dev breaks LCP into TTFB, resource load delay, load duration, and render delay to localise the bottleneck.
- Real user monitoring (RUM) metrics
Real user monitoring (RUM) measures web performance from actual visitors' browsers in the field, as opposed to synthetic lab testing in a controlled environment. Its headline metrics are the Core Web Vitals — Largest Contentful Paint, Interaction to Next Paint, and Cumulative Layout Shift — collected via the browser's performance APIs. Field data reflects real devices and networks, so it varies far more than lab numbers.
- Website observability
Watch timing signals next to human traffic.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.