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I understand there is a new perf metric that's available in Lighthouse called Perceptual Speed Index. (see info here https://github.com/pmdartus/speedline/) This is available from Chrome Dev Tools timeline. Wondering if this would be available in WPT?
Not currently. It's a variant of the Speed Index code that uses SSIM instead of histograms for checking completeness.

The main difference is that it is sensitive to page elements moving (i.e. the page shifting down a few pixels would radically change the completeness %). That could be a good or a bad thing depending on how you felt about layout moving.
(05-16-2017 11:07 PM)pmeenan Wrote: [ -> ]Not currently. It's a variant of the Speed Index code that uses SSIM instead of histograms for checking completeness.

The main difference is that it is sensitive to page elements moving (i.e. the page shifting down a few pixels would radically change the completeness %). That could be a good or a bad thing depending on how you felt about layout moving.

+1 to Pat's explanation. If you a want more detailed explanation, you could check the following URLs:

Perceptual Speed Index:

http://www.parvez-ahammad.org/blog/perce...f-webpages

Section-2 of our ACM SigComm 2017 paper:

https://arxiv.org/pdf/1704.01220.pdf

The topic of defining better visual metrics is actively evolving - stay tuned for more updates!

- Parvez (@perceptPA)
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