Bug 308224 – extremely high INP presentation delay for bfcache restores

• Status: NEW
• Component: Page Loading

[Erwin Hofman]

Created attachment 478408 [details]
inp segmented by navigation type, including sub-parts and additional (histogram + percentile) charts

*Summary:*
We are observing extremely high INP values for `back_forward_cache` navigations on Safari (mobile + desktop ) in our RUM data, primarily caused by very large presentation delay.

For the same traffic, same pages, same percentile (P75 mobile), Google Chrome reports normal INP values, while Safari reports extreme outliers.
This appears related to presentation delay calculation during bfcache restores.

Might be but not sure if related to https://bugs.webkit.org/show_bug.cgi?id=305251 (as I don't see bfache being part of its description).

*Data comparison:*
Safari (mobile P75)
- navigate: 161 ms
- back_forward: 141 ms
- reload: 184 ms
- back_forward_cache: 11013 ms

For back_forward_cache, ~41% of sessions fall into the worst bucket (>= 500ms), with extreme values dominating P75.

Chrome (mobile P75, same traffic sample size scale)
- navigate: 64 ms
- back_forward: 55 ms
- reload: 65 ms
- back_forward_cache: 53 ms

Chrome shows bfcache restores behaving as expected, with very low INP.

[Radar WebKit Bug Importer]

<rdar://problem/171273031>

[Franco Vieira de Souza]

Pull request: https://github.com/WebKit/WebKit/pull/59835