Misreporting aggregate script transfer size for repeated iframes loading same JS assets

[westonruter]

Provide the steps to reproduce

1. Run LH on https://ma.tt or any other page that has multiple YouTube players on it.

What is the current behavior?

Lighthouse is reporting a transfer size of ~3MB for YouTube scripts on a page that has give YouTube player iframes on it:

Each iframe has about 600KB of script.

What is the expected behavior?

Since the 600KB of script is loaded 5 times, then only 600KB should be reported as the transfer size.

Environment Information

• Affected Channels: PSI
• Lighthouse version: 6.0.0

[brendankenny]

It looks like this may be a PSI/LR bug? (and/or correctly reporting what's really happening during PSI's load but bug in that it differs from how a local load would go)

I can repro in PSI, but see this correct-looking transfer size in CLI/DevTools:

And from the 'network-records' hidden audit, can confirm that there's 45 requests to a youtube origin but 34 of those have a transferSize of 0 because they're loading from cache.

[brendankenny]

Also from the main-thread blocking time column PSI vs my results, it seems like all the youtube iframes may be running in-process on PSI vs out of process locally?

[patrickhulce]

Maybe frames are the source of all those unexplained duplicate requests in PSI!

If PSI isn't aware of requests between frames I could certainly see that being the case.

[connorjclark]

prob same issue as #8990