performance-impls: LCP should use startTime #35070
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Fixes #34383 |
rcebulko
reviewed
Jun 29, 2021
erwinmombay
approved these changes
Jun 29, 2021
Co-authored-by: Ryan Cebulko <[email protected]>
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Hey @jridgewell! These files were changed: |
rcebulko
approved these changes
Jun 29, 2021
jridgewell
requested changes
Jun 29, 2021
| if (entry.renderTime) { | ||
| this.largestContentfulPaintRenderTime_ = entry.renderTime; | ||
| } | ||
| this.largestContentfulPaint_ = entry.startTime; |
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Do all browsers support startTime?
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Yes, this is also how the web-vitals package implements it.
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@jridgewell: I've updated TICKEVENTS.md in 79da311
jridgewell
approved these changes
Jun 30, 2021
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summary
There isn't much utility in separately tracking LCP Load and LCP Render.
loadTimecorresponds to when the resource associated with a node has been loaded from network. It is always available on every node, but is relatively meaningless for any non-remote resource on a page. For example, all<h1>tags have a loadTime, but that time is 0. Therefore if a text node were chosen as the LCP Element, we'd report 0 for LCP-L.renderTimecorresponds to when the node is painted to the screen. This has a meaningful value for text nodes, but ends up being absent for any crossorigin resource without aTiming-Allow-Originheader. In these cases it is recommended to fallback to loadTime since the numbers will be relatively close to each other. Since AMP Caches usually host all their images, this is also not a very strong factor for AMP.cc @ampproject/wg-performance