What is Time to First Byte?
Time to First Byte (TTFB) is the elapsed time from starting a request or navigation until the first byte of the response arrives. Depending on where the timer starts, it can include redirect, DNS, connection, and TLS setup in addition to server processing.
How Time to First Byte works
TTFB is observed at the boundary between request setup and response arrival, so its value depends on where the timer begins and whether an existing connection is reused. Name resolution, TCP and TLS setup, proxy traversal, origin computation, and propagation can all contribute. In a media delivery pipeline, it helps localize startup delay before a manifest, segment, image, or API response begins transferring, but says nothing about the remaining payload.
Key facts
- 1Browser page-load instrumentation measures TTFB from navigation start, or from activation start for prerendered pages, to the response-start timestamp. This includes redirect, DNS, connection, and TLS time. A timer anchored at request start excludes that setup, so tool definitions must match before values are compared.
- 2An edge-cache hit can shorten TTFB by avoiding an origin round trip, while a miss may include shield, origin, database, or transformation work even when the response body is identical.
- 3Server-Timing data can expose selected backend phases alongside TTFB, but it cannot account for every client-side DNS, connection, proxy, and last-mile delay on the delivery path.
When Time to First Byte matters
Compare TTFB across cached and uncached requests to distinguish origin work from delivery-path latency. A low TTFB does not guarantee a fast complete response because download and rendering time remain separate.
Common use cases for delivery
These examples cover delivery broadly, not specifically Time to First Byte.
- Serving image, audio, video, and document derivatives to a geographically distributed audience.
- Protecting private assets worldwide with expiring or signed requests.
- Reducing repeated processing and origin traffic by caching deterministic results.
Working with delivery
This guidance covers delivery broadly, not just Time to First Byte.
A client requests an asset using a URL or playback manifest. A delivery layer evaluates authorization and cache state, serves a cached response when possible, or retrieves the asset from its origin before forwarding and optionally caching it.
Delivery choices determine more than download speed. Cache keys, origin behavior, authorization, geographic routing, invalidation, and egress cost decide whether an asset is fast, current, and available to the right audience.
What you gain
- Edge caching places frequently requested assets closer to viewers.
- Explicit cache and authorization rules reduce avoidable origin work.
- Multiple delivery variants let clients request an asset suited to their context.
What it costs
- Long cache lifetimes improve hit ratio but make replacement and invalidation more difficult.
- Signed access protects private media but adds key management, clock, and cache-partitioning concerns.
- More variants improve client fit while increasing storage, cache fragmentation, and operational complexity.
Before production
- 1Define cache keys, cache lifetime, invalidation, and authorization behavior explicitly.
- 2Measure time to first byte, cache-hit ratio, egress, and behavior after an origin failure.
- 3Test signed and unsigned requests at the CDN edge, not only against the origin.