Add Perceptual Speed Index to Required Metrics

[kristoferbaxter]

@addyosmani and @housseindjirdeh – We should consider adding PSI to the required metrics with a link explaining what it is.

Frequently implementations can trade-off faster "boot" times for degraded PSI and vice-versa. Since the goal is to understand the implementations (while we work on standardization of features), this metric could help visitors gain more insight into the motivations of each implementation.

[housseindjirdeh]

🙌 I think that's a great idea, will be more then happy to include it in the site with a snippet/link explaining what it is.

[addyosmani]

@kristoferbaxter @housseindjirdeh I'm more than happy for us to include PSI in the required metrics.

Link wise, we've got a brief explainer here: https://developers.google.com/web/tools/lighthouse/audits/speed-index and https://www.instartlogic.com/blog/perceptual-speed-index-psi-measuring-above-fold-visual-performance-web-pages

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At some point (no rush) we might find value in exploring an automated pipeline for periodically re-running our WebPageTest reports for each app and extracting the relevant scores we want to track from the WPT API. I believe what's included from the Lighthouse report can also be extracted from there, but we can chat more about whether this is something we want to do at a later point 😄

[housseindjirdeh]

@addyosmani I think that's a great idea and I've been thinking that we may definitely want to automate this sometime. Opened #71 to track it :)

[kristoferbaxter]

I'll see about collecting a new set of data including PSI for a branch.

[addyosmani]

Perfect. Thanks both!