First Interactive, also known as first CPU idle, is a non-standard web performance metric that measures when the user's window is quiet enough to handle user input, or what is termed as minimally interactive.
Minimally interactive is defined as when some, but not necessarily all, UI elements on the page have loaded and are interactive, and, on average, respond to user input in a reasonable amount of time.
First interactive is a variation of Time to Interactive, which is split into First Interactive and Consistently Interactive. These metrics have been proposed by the Web Incubator Community Group and are already being used in various tools. This metric has been called First CPU Idle since Lighthouse 3.0