Lighthouse metrics wrong

Question

I've got 0 performance-wise after running Lighthouse. I checked the metrics and here's what I saw:

First Contentful Paint 20.8 s
Speed Index 33.9 s
Time to Interactive 94.1 s
First Meaningful Paint 20.8 s
First CPU Idle 20.8 s
Max Potential First Input Delay 2,840 ms

But these metrics are absolutely wrong, the whole lighthouse check ran for 13s. How can it have measured that the first contentful paint was 20s in ??

Any idea why this would happen ? I'm running lighthouse in my Chrome dev tool. This is running on a netlify cdn website (protected with htaccess so maybe that's why ? Although I got the same problem running lighthouse in localhost)

Thanks in advance

Answer 1

By default, Lighthouse loads the page with the host network speed and CPU, and in a post-processing step does a simulation on a simplified model of the page load with slower network/CPU applied. See https://github.com/GoogleChrome/lighthouse/blob/master/docs/throttling.md

There are options to disable this throttling in the CLI and in DevTools.