How is duration computed for long main thread tasks?

[darahak-datadome]

Hi there,

I'm using Lighthouse in Chrome DevTools and I focus on the Performance report that signals long main thread tasks.
Inside the report page, I press the View Original Trace so I can identify what is causing those long tasks, but the numbers don't seem to match.

For example, Lighthouse will report a task that is running for more than 70ms.
When I look at the charts from the original trace, the longest task is only about 20ms and there is nothing in the Long tasks chart either.

Could someone clarify how the report should be interpreted and if there is an issue with the results?
Thank you.

[connorjclark]

see https://github.com/GoogleChrome/lighthouse/blob/master/docs/throttling.md#devtools-lighthouse-panel-throttling

short answer: Lighthouse does a simulation by default, that trace is not expected to match (that's why it's called Original Trace)