Consider to expose decoding time for FCP and LCP?

[sefeng211]

I haven't get a chance to do a proper testing for this, so I'll just refer to Michal's slide for TPAC 2022. Based on the slides, Firefox and Safari doesn't wait for the image to be decoded for FCP, but Chrome does.

Should we update the spec for FCP to wait for the image to be decoded?

I think the spec for LCP only waits for the size of the image to be known, it also doesn't wait for the image to be decoded. Should it also be updated as well?

There's a concern about leaking image decoding time. On one hand, image.decode() can leak the same information, however on the other hand, image.decode() uses a promise, so engines can add arbitrary delays to avoid leaking the same information.

We have some related discussions w3c/largest-contentful-paint#111, WICG/element-timing#65

[noamr]

This is also connected to #62.