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
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.
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.
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.
