debug-optimize-lcp

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. navigate_page with pageId to the target URL.
2. performance_start_trace 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.

Step 2: Analyze LCP Insights

Use performance_analyze_insight to drill into LCP-specific insights. Look for these insight names in the trace results: