Why it matters
LCP is when the main content appears. Past 2.5 s on mobile users perceive the page as slow, bounce more, and Google's Core Web Vitals mark the URL as needing improvement.
How to fix it
Make the LCP element discoverable in the initial HTML, never lazy-load it, give an LCP image fetchpriority="high" and a modern format, keep TTFB low (static/CDN), and avoid render-blocking CSS/JS before it.
Example
<img src="/hero.avif" width="1200" height="630" fetchpriority="high" alt="…">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.
cls-highCumulative Layout Shift is highttfb-slowServer response (TTFB) is slowfcp-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