✨ feat: collect LCP subparts RUM-13818

[HugoGarrido]

Motivation

LCP Subparts are values that compute the final LCP measure the browser SDK.

This is helpful for troubleshooting LCP since it help to define what type of LCP issue we are facing (server, render etc.) by looking at the biggest contributor (more details in this doc)

Here is a breakdown of each part :

Subpart	Meaning
TTFB	Already tracked by the Browser SDK as firstByte
Resource load delay	Time between firstByte and the loading start of the resource associated with the LCP. 0 if no resource are attached to the LCP
Resource load duration	Time to takes to load the resource attached to the LCP0 if no resource are attached to the LCP
Element render delay	Time between the LCP resource finishes loading and the LCP element is fully rendered

This PR adds the collection of the LCP subparts when we collect the LCP

Changes

• In trackLargestContentfulPaint when we have an LCP entry :
  • re-measure the TTFB (keeping the name firstByte)
  • get the performance entry related to the LCP resource if applicable
  • compute an optional subParts object that is added in the final payload
• Extract a helper to safely compute the firstByte value
• Extract a helper to safely read performance global
• Update tests, fixtures and types to support the new subParts object
• Add code behind COLLECT_LCP_SUBPARTS

Test instructions

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	167.98 KiB	168.61 KiB	+648 B	+0.38%	✅
Rum Profiler	4.31 KiB	4.31 KiB	0 B	0.00%	✅
Rum Recorder	24.54 KiB	24.54 KiB	0 B	0.00%	✅
Logs	56.25 KiB	56.29 KiB	+46 B	+0.08%	✅
Flagging	944 B	944 B	0 B	0.00%	✅
Rum Slim	124.88 KiB	125.49 KiB	+625 B	+0.49%	✅
Worker	23.63 KiB	23.63 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base CPU Time (ms)	Local CPU Time (ms)	𝚫%
RUM - add global context	0.0039	0.0047	+20.51%
RUM - add action	0.0129	0.0162	+25.58%
RUM - add error	0.0133	0.0158	+18.80%
RUM - add timing	0.0026	0.003	+15.38%
RUM - start view	0.0031	0.0049	+58.06%
RUM - start/stop session replay recording	0.0007	0.001	+42.86%
Logs - log message	0.0176	0.0197	+11.93%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	28.70 KiB	26.60 KiB	-2.09 KiB
RUM - add action	100.44 KiB	50.99 KiB	-49.45 KiB
RUM - add timing	27.83 KiB	27.66 KiB	-175 B
RUM - add error	102.86 KiB	54.29 KiB	-48.57 KiB
RUM - start/stop session replay recording	26.75 KiB	26.25 KiB	-512 B
RUM - start view	438.38 KiB	430.16 KiB	-8.22 KiB
Logs - log message	48.20 KiB	46.61 KiB	-1.59 KiB

🔗 RealWorld

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage
• Patch Coverage: 70.00%
• Overall Coverage: 77.23%

View detailed report

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 989e08d | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

[HugoGarrido]

I have read the CLA Document and I hereby sign the CLA

[chatgpt-codex-connector]

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Reviewed commit: 94434b0253

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[chatgpt-codex-connector]

Match the LCP resource to the correct request

The LCP resource timing is selected via getResourceEntries()?.find((e) => e.name === resourceUrl), which returns the first entry with that URL. Resource entries persist for the whole page and can include multiple requests for the same URL (e.g., preloads, cache-busting reloads, or SPA route changes). In those cases the first entry is often unrelated to the LCP element, so loadDelay/loadTime/renderDelay are computed from stale timings, corrupting the subparts metrics. Consider picking the most recent matching entry whose timing precedes lcpEntry.startTime instead of the first match.

Useful? React with 👍 / 👎.

[HugoGarrido]

/to-staging

[gh-worker-devflow-routing-ef8351]

View all feedbacks in Devflow UI.

2026-02-02 13:55:15 UTC ℹ️ Start processing command /to-staging

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2026-02-02 13:55:23 UTC ℹ️ Branch Integration: starting soon, merge expected in approximately 0s (p90)

Commit 989e08daac will soon be integrated into staging-06.

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2026-02-02 14:07:33 UTC ℹ️ Branch Integration: this commit was successfully integrated

Commit 989e08daac has been merged into staging-06 in merge commit 722f4b6fee.

Check out the triggered pipeline on Gitlab 🦊

If you need to revert this integration, you can use the following command: /code revert-integration -b staging-06

[HugoGarrido]

Code is deployed on staging for you to test!