Core Web Vitals triage

Lab tools give a synthetic score; the ranking signal comes from field data collected from real visitors. This skill works from field data down to the specific element causing each failure.

The skill file

---
name: core-web-vitals-triage
description: Use when field data shows failing LCP, INP or CLS and you need to know which fix will actually move the metric rather than guessing from a lab score.
---

# Core Web Vitals triage

Lab tools give a synthetic score; the ranking signal comes from field data collected from real visitors. This skill works from field data down to the specific element causing each failure.

## What you need first

- CrUX or Search Console Core Web Vitals report
- A lab trace (Lighthouse or WebPageTest) for a representative URL per template

## Method

1. Group failing URLs by template, not individually - one template fix usually resolves thousands of URLs.
2. For LCP, identify the LCP element per template from the trace. It is almost always a hero image, a webfont-rendered heading, or a client-rendered block.
3. Split the LCP into its four parts: time to first byte, resource load delay, resource load time, render delay. Fix the largest part, not the one that is easiest.
4. For INP, capture interactions with the longest processing time and look for long tasks on the main thread - usually third-party scripts or a heavy hydration step.
5. For CLS, find what shifts: images without dimensions, injected banners, late-loading webfonts without a matched fallback metric.
6. Verify against field data after 28 days, since that is the collection window - a lab improvement that field data never confirms is not a fix.

## What this produces

Per template: the failing metric, the specific element responsible, the breakdown that proves it, and the change to make.

## Where this goes wrong

- Optimising to a lab score while field data is unchanged
- Fixing the smallest LCP sub-part because it is the easiest
- Judging results before the 28-day field window has rolled over

---

From the QuQi skill library - https://www.quqi.io/skills/core-web-vitals-triage

What you need first

• CrUX or Search Console Core Web Vitals report
• A lab trace (Lighthouse or WebPageTest) for a representative URL per template

Method

1. 01 Group failing URLs by template, not individually - one template fix usually resolves thousands of URLs.
2. 02 For LCP, identify the LCP element per template from the trace. It is almost always a hero image, a webfont-rendered heading, or a client-rendered block.
3. 03 Split the LCP into its four parts: time to first byte, resource load delay, resource load time, render delay. Fix the largest part, not the one that is easiest.
4. 04 For INP, capture interactions with the longest processing time and look for long tasks on the main thread - usually third-party scripts or a heavy hydration step.
5. 05 For CLS, find what shifts: images without dimensions, injected banners, late-loading webfonts without a matched fallback metric.
6. 06 Verify against field data after 28 days, since that is the collection window - a lab improvement that field data never confirms is not a fix.

What this produces

Per template: the failing metric, the specific element responsible, the breakdown that proves it, and the change to make.

Where this goes wrong

• Optimising to a lab score while field data is unchanged
• Fixing the smallest LCP sub-part because it is the easiest
• Judging results before the 28-day field window has rolled over

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