Agent skill · data analytics · chromedevtools
debug-optimize-lcp
Guides debugging and optimizing Largest Contentful Paint (LCP) using Chrome DevTools MCP tools. Use this skill whenever the user asks about LCP performance, slow page loads, Core Web Vitals optimization, or wants to understand why their page's main content takes too long to appear. Also use when the user mentions "largest contentful paint", "page load speed", "CWV", or wants to improve how fast their hero image or main content renders.
Why this skill is useful
Provides specific debugging workflows and commands for optimizing LCP that the AI wouldn't reliably generate on its own.
What it needs
About 4k tokens when loaded. Last updated 2026-08-06. 48,655 stars on the source repository.
What this skill does
What is LCP and why it matters Largest Contentful Paint (LCP) measures how quickly a page's main content becomes visible. It's the time from navigation start until the largest image or text block renders in the viewport. Good: 2.5 seconds or less Needs improvement: 2.5–4.0 seconds Poor: greater than 4.0 seconds LCP is a Core Web Vital that directly affects user experience and search ranking. On 73% of mobile pages, the LCP element is an image. LCP Subparts Breakdown Every page's LCP breaks down into four sequential subparts with no gaps or overlaps. Understanding which subpart is the bottleneck is the key to effective optimization. Subpart Ideal % of LCP What it measures ----------------------------- -------------- ---------------------------------------------- Time to First Byte (TTFB) ~40% Navigation start → first byte of HTML received Resource load delay <10% TTFB → browser starts loading the LCP resource Resource load duration ~40% Time to download the LCP resource Element render delay <10% LCP resource downloaded → LCP element rendered The "delay" subparts should be as close to zero as possible. If either delay subpart is large relative to the total LCP, that's the first place to optimize. Common Pitfall: Optimizing one subpart (like compressing an image to reduce load duration) without checking others. If render delay is the real bottleneck, a smaller image won't help — the saved time just shifts to render delay. Debugging Workflow Follow these steps in order. Each step builds on the previous one. Step 1: Record a Performance Trace Navigate to the page, then record a trace with reload to capture the full page load including LCP: 1. navigatepage with pageId to the target URL. 2. performancestarttrace with pageId, reload: true and autoStop: true. The trace results will include LCP timing and available insight sets. Note the insight set IDs from the output — you'll need them in the next step. …
How to use it
Reference it in AdaL, Claude Code, Cursor or any coding agent — nothing to install:
@skills chromedevtools/debug-optimize-lcp