Add LCP changelog for cross-origin renderTime change

Change-Id: Id0b43cad7c6f9858592be979140513d59805122c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6339853
Reviewed-by: Noam Rosenthal <nrosenthal@chromium.org>
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Barry Pollard <barrypollard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1431504}

docs/speed/metrics_changelog/2025_02_lcp.md

@@ -0,0 +1,30 @@
+# Largest Contentful Paint Changes in Chrome 133
+
+## Expose coarsened cross-origin renderTime in element timing/LCP (regardless of TAO)
+
+Prior this change all cross-origin element-timing and LCP performance entries
+would have a zero renderTime, unless they were served with the appropriate
+Timing-Allow-Origin HTTP header to allow access to detailed timings. Without a
+renderTime LCP falls back ot the load time for LCP which can often be well
+before the load time. This can result in anomalies such as LCP appearing to
+happen before FCP, which is of course not possible.
+
+With this change presentation timestamps (renderTime, paint timing start time,
+event timing end time) are instead just coarsened to a 4ms multiple to mitigate
+the risk of reading cross-origin image information.
+
+For more details see the [Chrome Status entry](https://chromestatus.com/feature/5128261284397056)
+
+### How does this affect a site's metrics?
+
+User may notice an increased LCP time in Real User Monitoring (RUM) metrics.
+The issue did not affect the Chrome User Experience Report (CrUX), and this was
+one example of [differences between CrUX and RUM](https://web.dev/articles/crux-and-rum-differences#cross-origin_resources).
+
+The LCP is now more accurate and more in line with CrUX so this is not a
+performance regression, but more an improvement to measurements of LCP.
+
+### When were users affected?
+
+Launch was the week of Feb 27, 2025, when m133 was in stable, but a limited
+number of early adopters might have seen this change as early as m132.

docs/speed/metrics_changelog/lcp.md

@@ -2,6 +2,8 @@
 
 This is a list of changes to [Largest Contentful Paint](https://web.dev/lcp).
 
+* Chrome 133
+  * Implementation optimizations: [Expose coarsened cross-origin renderTime in element timing/LCP (regardless of TAO)](2025_02_lcp.md)
 * Chrome 130
   * Launch feature: [Exclude transparent text from being LCP eligible](2024_10_lcp.md)
 * Chrome 126