What Are the Most Common Pitfalls That Increase First Input Delay (FID) and How Can They Be Avoided During Website Development?


First Input Delay (FID) measures the time from when a user first interacts with a page (e.g., by clicking a link) to the time when the browser is able to begin processing event handlers in response to that interaction. Common pitfalls that increase FID include heavy JavaScript execution, long-running tasks, and render-blocking resources. This guide provides strategies to mitigate these issues and improve FID.

Heavy JavaScript Execution

Minimize and Optimally Load JavaScript

Excessive JavaScript execution can significantly increase FID. By minimizing and deferring non-essential JavaScript, you can reduce the amount of work the main thread needs to do before it can respond to user interactions.


Long-Running Tasks

Break Up Long Tasks

Long tasks block the main thread and prevent it from being able to respond to user interactions in a timely manner. It's essential to identify and break up long-running tasks into smaller chunks.


Render-Blocking Resources

Optimize CSS and Font Loading

Render-blocking resources, such as CSS and web fonts, can delay the time it takes for the page to be interactive.


Reduce Server Response Times

Improving server response times can influence FID by ensuring that the browser receives resources quickly and can start processing them sooner.


Reduce Main-Thread Work

Efficiently Process Third-Party Scripts

Third-party scripts can often introduce performance bottlenecks. It’s important to monitor and manage these scripts carefully.



Improving First Input Delay (FID) involves a combination of minimizing heavy JavaScript execution, breaking up long tasks, handling render-blocking resources efficiently, improving server response times, and managing third-party scripts. Applying these strategies will enhance the responsiveness of your website and offer a better user experience.