What is First Input Delay?
First Input Delay (FID) is a Core Web Vital metric that measures the time between when a user first interacts with your webpage – clicking a button, typing in a form, or tapping a link – and when the browser actually responds to that interaction.
Unlike page load speed, FID captures real-world responsiveness. A user might see your page load quickly, but if the main thread is busy executing JavaScript, their click won't register immediately. That delay is FID.
Why FID Matters for UK Media Buyers
Google integrated FID into its ranking algorithm, making it a direct SEO factor. For media agencies managing paid search and display campaigns, poor FID means:
- Higher bounce rates on landing pages, wasting ad spend
- Lower conversion rates when users abandon forms due to lag
- Poor ad performance metrics, affecting ROI reporting to clients
- Competitive disadvantage if competitors optimise their sites better
In competitive UK markets – particularly finance, e-commerce, and SaaS – milliseconds matter. A 200ms delay can significantly impact campaign performance.
How FID Works
FID specifically measures the delay, not the interaction duration. For example: - User clicks a "Book Now" button at 1 second - Browser responds at 1.15 seconds - FID = 150 milliseconds
Google's thresholds are: - Good: 0–100ms - Needs improvement: 100–300ms - Poor: 300ms+
When You'll Use FID Data
When auditing landing pages for your campaigns, particularly those driving conversions. FID issues typically stem from:
- Heavy JavaScript execution
- Third-party scripts (trackers, ads, chat widgets)
- Main thread blocking
FID vs CLS vs LCP
FID is one of three Core Web Vitals. Largest Contentful Paint (LCP) measures visual load speed, while Cumulative Layout Shift (CLS) tracks visual stability. All three affect rankings.
Optimisation Basics
Work with developers to: - Defer non-critical JavaScript - Break up long JavaScript tasks - Audit third-party script impact - Use web workers for heavy processing
Monitor FID in Google Search Console, PageSpeed Insights, or tools like Contentsquare to track improvements over time.