First CPU Idle Time measures how long it takes a page to become minimally interactive during its loading sequence.
This is important as it can be impacted by rich media such as videos that play on after a page loads, tying up thread time.
How First CPU Idle Time Impacts User Experience & SEO Rankings
When a page is loading, files are being transferred & opened by the browser to come up with the page’s rendering & interactive bits.
This requires a great deal of work on the backend that the end user cannot see.
Once enough of these processes have taken place & the page has rendered & the interactive elements are loaded, there is more thread space freed up, which enables the users to now interact with the site’s content.
This is an important metric to optimize for, as users do not want to be waiting to interact with content on your webpages.
The longer it takes for a user to be able to read or interact with your site’s content, the more likely they are to bounce to another source to find their answers, and less likely they are to convert.
Part of how Google & other search engines rank websites includes data on how long it takes for pages to load & for users to interact with them, as search queries are meant to mimic spoken queries between people.
Therefor it is essential that all content load in as much time & few requests as possible, so that users can interact with it sooner.