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TTFB
This post introduces the Time to First Byte (TTFB) metric and explains how to measure it.
Some servers allow flushing of the document response to happen before the main body is available—either with just the HTTP headers, or with the<head> element, both of which could be considered similar in effect to Early Hints and so also cloud the definition of what TTFB measures. However, it will be affected by factors outside of that direct control such as redirect time, whether it is served from a cache hit by a CDN or has to make a potentially longer journey back to an origin server. Conversely, a server-rendered site that does not require as much client-side work could have a higher TTFB, but better FCP and LCP values than an entirely client-rendered experience.
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