files/en-us/glossary/time_to_first_byte/index.md
Time to First Byte (TTFB) refers to the time between the browser requesting a page and when it receives the first byte of information from the server. This time includes {{Glossary("DNS")}} lookup and establishing the connection using a {{Glossary("TCP")}} handshake and {{Glossary("TLS")}} handshake if the request is made over {{Glossary("HTTPS")}}.
TTFB is the time it takes between the start of the request and the start of the response, in milliseconds. This can be measured using the {{domxref("PerformanceResourceTiming.responseStart", "responseStart")}} attribute of {{domxref("PerformanceNavigationTiming")}}:
const ttfb = performance.getEntriesByType("navigation")[0].responseStart;
[!NOTE] For sites using {{HTTPStatus("103", "103 Early Hints")}}, TTFB is typically the first bytes (after any redirects) — and so, the 103 interim response. Site owners wishing to measure the time until the final response should use {{domxref("PerformanceResourceTiming.finalResponseHeadersStart", "finalResponseHeadersStart")}}, where supported.