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Answer: TTFB is the time from the start of navigation until the first byte of the server's response arrives. It is a good signal that a web server is slow to respond. (S1)

Details

• For page navigations, TTFB comes before every other meaningful loading metric. (S1)
• In browser timing terms it runs from startTime to responseStart, and it adds up phases such as redirects, service worker startup, and DNS lookup. (S1)
• 103 Early Hints responses complicate what counts as the "first byte"; web.dev explains how Chrome now treats them. (S1)

Thresholds

• A rough target is 0.8 seconds or less. (S2)
• A slow TTFB pushes back every metric that follows it. (S2)
• TTFB is not a Core Web Vital, so missing its "good" mark is acceptable as long as it does not hurt the metrics that do count. (S2)
• A server-rendered page with less client-side work can have a higher TTFB yet better FCP and LCP than a fully client-rendered one. (S2)

Claims

• id: C1 claim: TTFB adds up several request phases, such as redirects and DNS lookup. source: S1 evidence: "TTFB is the sum of the following request phases"
• id: C2 claim: TTFB is startTime to responseStart. source: S1 evidence: "TTFB measures the elapsed time between startTime and responseStart."
• id: C3 claim: Aim for 0.8 seconds or less. source: S2 evidence: "most sites should strive to have a TTFB of 0.8 seconds or less."
• id: C4 claim: TTFB is not a Core Web Vital. source: S2 evidence: "TTFB is not a Core Web Vitals metric"

Sources

• [S1] Time to First Byte (TTFB)  |  Articles  |  web.dev. Google (web.dev). https://web.dev/articles/ttfb . License: CC BY 4.0 (web.dev live page footer: 'the content of this page is licensed under the Creative Commons Attribution 4.0 License'; cite the page, not the archived repo). Accessed 2026-11-05.
• [S2] Optimize Time to First Byte  |  Articles  |  web.dev. Google (web.dev). https://web.dev/articles/optimize-ttfb . License: CC BY 4.0 (web.dev live page footer: 'the content of this page is licensed under the Creative Commons Attribution 4.0 License'; cite the page, not the archived repo). Accessed 2026-11-05.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_zc08p3sn,
  author = {{iinventedlight}},
  title = {{Time to First Byte (TTFB)}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/webdev-ttfb/},
  note = {Permanent ID iil:note:zc08p3sn. License: CC-BY-4.0.},
}

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