1889717 - LCP timestamp on https://essence-theme-bold.myshopify.com/ is to early

Status: NEW

(Core :: DOM: Performance APIs, defect)

Description

[Jeff Muizelaar [:jrmuizel]]

When loading https://essence-theme-bold.myshopify.com/ LCP timestamp is too early.
See this profile: https://share.firefox.dev/43MSL43

Chrome's timestamp seems to accurately match reality: https://share.firefox.dev/4aIIB6C and https://trace.cafe/t/dzWWTRUe68

Comment 1

[Sean Feng [:sefeng]]

Are we talking about the LCP timestamp at the 91ms? This is how we get the timestamp and I think it's the timestamp of when we finish building the displaylist for this presShell. What Chrome reports seem to align with FirstContentComposite which is more closer to the actual paint.

I think this difference is expected, am I misunderstanding something?

Comment 2

[Jeff Muizelaar [:jrmuizel]]

Yep, I'm talking about the LCP at 91ms. If we're using the timestamp of when we finish building it seems like we're taking the timestamp too early. We should move it later so that it includes composition. I'll file a separate bug for that.

The other problem then seems to be that we're trigger LCP before we've decoded the large image. So instead of having a timestamp around 91ms it should be closer to 200ms. Sean, do you know why we're reporting it before the image is done decoding?

Comment 3

[Sean Feng [:sefeng]]

We discussed this on Matrix. Not including composition time is intentional and matching the spec. We'd need to do more spec works and gaining consensus from other engines if we want to include it.

Filed https://github.com/w3c/paint-timing/issues/104 for exposing image decoding time.

Comment 4

[BugBot [:suhaib / :marco/ :calixte]]

The severity field is not set for this bug.
:fdoty, could you have a look please?

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