Vitals from Lighthouse VS Vitals from Original Trace

[Lionel Enkaoua]

I'm trying to understand the big difference in Vitals displayed in Lighthouse results compared to Vitals displayed in Original Trace.

For example on mobile in Lighthouse in incognito I see:

FCP: 1.0s
TBT: 0ms
SI: 1.0s
LCP: 2.7s

CLS: 0

And in original Trace:
LCP, FP, FCP: 371.62 ms

Also I have other vitals numbers if I run Performance in incognito too with:
- CPU throttling on: 4x slowdown
- Network: Slow 4G (DL: 1638.4KB/s, UL: 750KB/s, 150ms for RTT)
- Mobile with screen resolution: 412 x 823

FP, FCP: 1.10s
LCP: 1.50s

What am I missing ?

The website i'm testing is not on my machine, onregular server, and I'm using Chrome 115.0.5790.170 on Ubuntu

[Barry Pollard]

This is best asked on the Lighthouse GitHub repo as it’s not feedback about the Web Vitals metrics themselves.

However, short answer is that Lighthouse uses its own throttling mechanism (

https://github.com/GoogleChrome/lighthouse/blob/main/docs/throttling.md). This allows a Lighthouse run to be relatively quick, but still give results as if it was on a slower network/device. The original trace is the unthrottled run.

Lighthouse throttling is also more complicated than a straight multiplier of the unthrottled metric numbers. Hence why FCP and LCP can diverge even if they are the same in the unthrottled trace (in the same way as they would likely diverge on a real slower device/slow network).

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[Barry Pollard]

Oh and FYI you can switch to use the DevTools throttling rather than Lighthouse throttling when running Lighthouse from Devtools:

And we're also changing the UX on this View Trace option to make this more obvious: https://github.com/GoogleChrome/lighthouse/pull/15315. This is included in Lighthouse v11.0.0 expected to be released in Chrome 118

[Lionel Enkaoua]

Awesome, thanks for the info!