First Input Delay (FID) is a critical performance metric in web development that measures the time between a user’s first interaction with a page—such as clicking a link, tapping a button, or other input—and the browser’s response. This metric reflects a webpage’s responsiveness and serves as a core indicator for assessing user experience. FID specifically tracks delays caused by the main thread being occupied with processing scripts or other tasks, directly affecting the user’s perception of interactivity.
A low FID value is crucial for delivering a smooth, engaging user experience, as it ensures near-instantaneous responses to user inputs. Significant delays can lead to frustration, site abandonment, or negative brand perception. Developers optimize FID by streamlining JavaScript execution, reducing intensive processing tasks, and prioritizing critical operations during initial page load. These improvements reduce delays and enhance overall web performance.
FID belongs to a broader set of metrics called Core Web Vitals, which search engines like Google use to evaluate website user experience quality. By analyzing first input delay, developers and site owners can identify performance bottlenecks and implement targeted improvements to boost interactivity and satisfaction. Continuous FID monitoring, alongside other performance metrics, helps maintain a competitive digital presence by ensuring seamless, responsive user experiences.
👉 See the definition in Polish: First Input Delay (FID): Opóźnienie pierwszej interakcji