Why it matters
Every millisecond before the first byte delays everything else. TTFB above ~800 ms usually makes a good LCP impossible.
How to fix it
Serve static or prerendered HTML from a CDN edge, cache dynamic HTML with s-maxage + stale-while-revalidate, stream the shell early, and remove redirects on the entry URL.
References
More performance rules
void perf reports 25 rules in this category. Core Web Vitals and bytes: LCP, CLS, TBT, TTFB, JavaScript, CSS, fonts, images, caching and compression, measured as the median of several runs on a throttled mobile and a desktop profile.
lcp-slowLargest Contentful Paint is slowcls-highCumulative Layout Shift is highfcp-slowFirst Contentful Paint is slowtbt-highMain thread is blocked by long tasks during loadjs-budgetFirst-load JavaScript exceeds the budgetcss-budgetCSS transfer exceeds the budgetfont-budgetWeb font transfer exceeds the budgettoo-many-fontsToo many font files are downloaded