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Time to First Byte (TTFB)

The short answer

TTFB measures how long a browser waits after requesting a page before the server sends back the very first byte of the response.

Full explanation

A slow TTFB — often caused by weak hosting, an unoptimized database, or no caching — delays every other metric downstream of it, since nothing else on the page can start loading until that first byte arrives.

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