Time to First Byte | Calibre Documentation

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SyntheticCrUXRUM

Time to First Byte (TTFB) is a a metric that reports the time between a request for a resource and the first byte arriving to the browser.

How is Time to First Byte calculated?#

Time to First Byte is a sum of several request and networking phrases:

redirect time
service worker startup time (if present)
DNS lookup
connection and TLS negotiation
the request (until the first byte has arrived)

Because a slow response time can have a cascading, adverse effect on overall web performance, we recommend optimising Time to First Byte as a priority.

What Time to First Byte measurement is fast?#

Pages with Time to First Byte below or equal 800ms are considered as fast, and above 1800ms are considered slow.

How to improve Time to First Byte?#

Time to First Byte covers everything between the request and the first byte of the response, so the levers are mostly on the server and the network path. Work through these steps:

1. Use a CDN and cache at the edge#

Serve from close to users: A Content Delivery Network shortens the network path between visitors and your content.
Cache responses: Cache full HTML responses at the edge where you can, so requests do not always reach the origin.

2. Speed up server-side processing#

Optimise the work per request: Tune slow database queries, add application or object caching, and avoid slow third-party calls during render.
Use faster delivery: Consider streaming responses or Early Hints (HTTP 103) so the browser can start work sooner.

3. Reduce redirects and connection overhead#

Remove redirect chains: Each redirect adds a round trip before the first byte can arrive.
Use modern protocols: Serve over HTTP/2 or HTTP/3, keep TLS efficient, and preconnect to required origins. The Round Trip Time of the network path sets a floor on Time to First Byte.

4. Monitor Time to First Byte over time#

Track TTFB with Calibre: TTFB is available in Real User Monitoring, the CrUX Dashboard and synthetic tests. Set a performance budget and segment by page to find the slowest routes.
Compare against thresholds: TTFB is good at 800ms or less and poor above 1800ms. See Core Web Vitals thresholds.