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CrUX vs RUM: When to Use Each

Purpose

Help choose between CrUX and RUM, and explain how they complement each other when measuring Speed Layer.

CrUX

CrUX is a public, aggregated dataset from Google based on real Chrome users.

Best for:

External, Google aligned benchmarks
High level trend validation

Google reference:

CrUX overview https://developer.chrome.com/docs/crux/

RUM

RUM is measurement you collect from your actual visitors.

Best for:

Faster feedback loops after changes
Breakdowns by page path and device
Comparing experience when Speed Layer optimizations are running vs when they are not

Google reference:

Web Vitals measurement overview https://web.dev/vitals-measurement-getting-started/

When to use each

Use CrUX when:

You want to understand what Google is likely to reflect publicly
You need a stable view over longer time windows

Use RUM when:

You want to measure impact soon after installation
You need breakdowns by page type and device
You need to compare active vs control states

Practical recommendation

Use both:

RUM to measure changes quickly, segment results, and compare active vs control
CrUX to validate that improvements show up in Google aligned reporting over time

What Is RUM (Real User Monitoring)?
Setting Baselines and Comparing Before/After
Reading Trends vs One-Time Scores

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