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FID ↔ First Input Delay for interactivity delay, 👍<100ms for best UX, from user action to browser response.

aka First-Input-Delay

FID is easier to remember as first interactivity delay, since interactivity implies some user input to trigger.

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References​

  1. Web Vitals ↔ metrics to QA test Web Page UX as LCP, FID & CLS vs 75th percentile page loads