SEO
Page experience diagnosis
Use when Core Web Vitals are failing and you need a prioritised engineering brief.
page-experience-diagnosis.md
You are diagnosing page experience problems and writing a brief engineers can act on.
Template or URL: {{URL}}
Field data (CrUX or RUM: LCP, INP, CLS, split by device): {{FIELD_DATA}}
Lab data (Lighthouse or WebPageTest summary): {{LAB_DATA}}
Stack: {{STACK}}
Known third party scripts: {{THIRD_PARTIES}}
Output:
1. A metric table: Metric | Field value | Threshold | Pass or fail | Device where it is worst.
2. For each failing metric, the likely causes ranked by probability, each with the evidence from the supplied data that supports it.
3. A fix table: Fix | Metric affected | Expected direction of change | Effort (S / M / L) | Risk of regression | Who does it.
4. "Measure this to confirm" - the exact check for each fix.
Constraints:
- Field data outranks lab data. Where they disagree, say which you are trusting and why.
- Do not predict a numeric improvement, for example "will cut LCP by 1.2s". State direction and reasoning instead.
- Call out any fix that improves lab scores without improving real user experience.
- If a third party in {{THIRD_PARTIES}} is a plausible cause, name it and say how to test by removal.
Fill in before running
Replace each placeholder with your own detail. The more specific you are, the less the model invents.
- {{URL}}
- {{FIELD_DATA}}
- {{LAB_DATA}}
- {{STACK}}
- {{THIRD_PARTIES}}
Getting a better result
-
1
Always paste field data - lab scores alone lead to optimising the wrong thing.
-
2
Split by device before pasting, since mobile INP failures usually have a different cause to desktop.
-
3
Ask for the measurement check to be added to the ticket so the fix can be proven.