Technical metric
Time to First Byte (TTFB)
Also: TTFB, server response time, first byte time
TTFB is mostly a server metric. A site can look quick in the browser and still answer slowly if every page is built on request without caching. Heavy page builders and uncached database queries are the usual causes.
The fixes are structural: cache pages, serve static HTML where possible, and deliver from a CDN edge. Make sure the content a crawler needs is in that first HTML response rather than loaded later by JavaScript.