TTFB (Time To First Byte)
TTFB measures the delay between the request and the first byte received: network latency plus server computation time. Under 200 ms is excellent; beyond 800 ms, the rest of the page queues behind it.
In practice
Its levers, in order of efficiency: serve from cache rather than compute, bring the server closer to the user (CDN), optimize the database and application code. A degraded TTFB propagates: LCP, however well optimized, never recovers from a late first byte. It is the first metric to examine on a slow site.