Performance trace vs Lighthouse Report

Hey guys! First I want to thank you for all your work. I’ve been using Lighthouse a lot the last year and a half and I’m just grateful of your work. So, straight to the question:

• Why the numbers/timings of a trace (generated using --save-assets) are different from the numbers in the Lighthouse report?

Example:

• FCP: 592.1
• LCP: 722.9

• FCP: 3800
• LCP: 6700

In this case FCP 6.4x is and LCP is 9.2x the trace value.

I suspect this might be linked to throttling (the trace is unthrottled?). If that is the case, is there a way to transform the unthrottled trace to a throttled trace?

The problem for us now is that we are seeing a high variance in the metrics over time.

And we are not sure why the scores in lighthouse change. When we compare the trace of a good result with a bad one, the main difference I see that FCP and LCP happens at the same time:

Other details

• Lighthouse v6.4.1 running in a docker container
• Docker Host is a Mac Mini
• Docker image based in debian: node:12-buster-slim
• Using puppeteer v5.5.0 (Chrome 88)
• BenchmarkIndex very consistent in the 900 range
• CPU Slowdown Multiplier of 2.2x

[dvelasquez]

Great! Thanks @patrickhulce

I manage to call the function and sending the artifacts of the lighthouse run. Thanks very much!

[patrickhulce]

@dvelasquez you can copy/paste and modify this function from the codebase to give yourself the traces instead. They’re effectively saved in global scope to the Simulator class.

https://github.com/GoogleChrome/lighthouse/blob/07b1f1b68ade8fde56556684fc0dff3704ff566b/lighthouse-core/lib/asset-saver.js#L243-L253