Time to first byte
TTFB · The time from the browser sending a page request to receiving the first byte of the response from the server; an indicator of server and network speed.
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Why it matters
Everything else on the page is added on top of this time; if the server responds late, even the best-optimised images cannot speed up the main content. Slow database queries, uncached dynamic pages and servers far from the user are common causes. Google considers 0.8 seconds or less good.
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Example
Every page on a news site is rebuilt from the database on every request, and the first response takes two seconds. Once pages are cached for a few minutes, the time to first byte drops to 200 milliseconds.
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Common mistake
Looking for slowness only on the front end. Image and code improvements made without measuring server response time may not have the expected effect.
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