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First Input Delay (FID) is a real user metric that measures the load responsiveness of a site and quantifies that data to reflect the experiences users have when interacting with unresponsive pages. A low FID indicates that the page is usable. For the ideal customer experience on your site the recommend threshold for First Input Delay is 100 milliseconds or less.
It is important to note that FID can only be collected through real user data. This metric is a reflection of actual human interaction on your site and cannot be measured with synthetic. Total Blocking Time (TBT), however, can be collected via synthetics and also measures the effects of interactivity.
Analyzing FID Data
For more information on how to analyze your FID data check out this page from web.dev.
If you are interested in how to optimize your FID check out this article by web.dev here.
processingStart attribute’s getter returns a timestamp captured at the beginning of the event dispatch algorithm. This is when event handlers are about to be executed.
startTime: This attribute MUST return the time value of the first recorded timestamp of this performance metric. If the startTime concept doesn't apply, a performance metric may choose to return a