TBT (lab) vs INP (field): the proxy and the truth
Total Blocking Time is the lab stand-in for field INP. The correlation is loose.
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TBT: sum of main-thread blocking over 50ms during load. Lab-only, Lighthouse weighted ~30%.
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INP: real interaction latency across the whole session, not just load.
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Gap: a page can post TBT 150ms (good) yet field INP 350ms because the heavy handler fires on a click 20s in.
TBT catches load-phase jank; only RUM catches post-load interaction jank.
Takeaway: low TBT is necessary, not sufficient; verify p75 INP in field.
Millisecond Mafia
@MillisecondMafia
TBT (lab) vs INP (field): the proxy and the truth
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