Time to First Byte: definition
Time to First Byte (TTFB) is the elapsed time from initiating a request until the client receives the first byte of the response. For a navigation, that interval can include redirect time, connection setup, TLS negotiation, request transmission, server processing and network delay on the return path.
What TTFB tells you
TTFB is an early-response metric. A high value means the browser waited a long time before response bytes started arriving, but the metric alone cannot identify whether the delay came from DNS, connection establishment, the origin, a cache miss, a reverse proxy or network distance.
Field data and synthetic measurements
Real-user measurements include the diversity of actual devices, networks, locations and cache states. Synthetic checks give you controlled repeatability from known locations. Both are useful: field data shows user impact while synthetic data can make regressions easier to reproduce.
Common causes of slow TTFB
- Slow application or database processing before the response starts.
- Cache misses or expensive server-side rendering.
- Network distance or routing changes between client and origin.
- Connection or TLS setup overhead when connections cannot be reused.
- Redirect chains that add extra request-response cycles.
How to investigate
Break the request into phases where your tooling allows it. Compare CDN or edge timing with origin timing, inspect server traces for the same request, and check whether slow TTFB clusters around a route, region, deployment or cache state. A single browser observation is useful for triage but not enough to characterize a production service.
TTFB is not full page speed
A fast first byte does not guarantee a fast page. The browser still has to download, parse and execute resources and render useful content. Conversely, a slower TTFB can sometimes coexist with acceptable user experience if the rest of the page is optimized. Use TTFB with other loading and interaction metrics.
Related concepts
Short answer
TTFB measures how long the client waits for the first response byte. It is useful for detecting early-request delay, but diagnosing the cause requires phase timing, server-side telemetry and network or cache context.
Practical checklist
- Record whether redirects are included in the observed navigation.
- Separate connection/network delay from origin processing when possible.
- Compare real-user field data with controlled synthetic tests.
- Do not treat TTFB as a complete page-experience metric.