Definition
FID (First Input Delay) measured the time between a user's first interaction with a page (clicking a button, tapping a link) and the moment the browser began processing that interaction. A high FID meant the browser was busy executing heavy JavaScript and could not respond immediately. Google recommended a FID below 100 milliseconds for a good experience. Since March 2024, FID has been replaced by INP (Interaction to Next Paint) as the official Core Web Vitals metric. INP is considered more representative because it measures all interactions during the session, not just the first. Sites that had optimized their FID may still have poor INP if subsequent interactions are slow.
Key Points
- Replaced by INP since March 2024
- Previous recommended threshold: under 100 milliseconds
- Main cause: heavy JavaScript blocking the main thread
Practical Examples
Blocking JavaScript
A site loads a 2 MB JavaScript file on load, causing a 350ms FID. After code splitting and async loading, FID drops to 50ms.
Third-party scripts
Adding 5 third-party ad scripts increases FID to 500ms. Deferring these scripts reduces FID to 80ms.
Frequently Asked Questions
No, since March 2024, Google has replaced FID with INP. INP is more representative because it measures responsiveness throughout the entire user session.
FID only measured the first interaction, while INP accounts for all interactions during the visit and retains the worst latency.
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Last updated: 2026-02-07