Why it matters
Until the first paint the user stares at a blank screen. FCP above 1.8 s feels broken on mobile.
How to fix it
Cut render-blocking CSS and synchronous head scripts, inline critical CSS for tiny pages, and use font-display: swap/optional.
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 highttfb-slowServer response (TTFB) 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