core: defer to trace engine for lcp-breakdown, ttfb #16733
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connorjclark
commented
Oct 7, 2025
| "speedIndex": 1583, | ||
| "speedIndexTs": 23467606130, | ||
| "timeToFirstByte": 565, | ||
| "timeToFirstByteTs": 23466588051, |
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This trace is just too old for the trace engine to handle wrt ttfb.
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For the observed metrics of ttfb and LCP subparts, stop using Lighthouse's original
ProcessedNavigationand instead use the CDT trace engine.I'm pretty sure these values are only used in hidden audits (metrics, predictive-perf).
I also changed the LCP breakdown result from being lcpLoadStart/End to now use the proper subpart names. These values are the phase durations rather than the "time to lcp load start" or "time to lcp load end". I also added the render delay phase, but that can't be simulated so it only appears for observed/provided throttling.