Total Blocking Time and Time To Interactive instead.
First CPU Idle is one of six metrics tracked in the Performance section of the Lighthouse report. Each metric captures some aspect of page load speed.
Lighthouse displays First CPU Idle in seconds:
First CPU Idle measures how long it takes a page to become minimally interactive. A page is considered minimally interactive when:
Both First CPU Idle and Time to Interactive measure when the page is ready for user input. First CPU Idle occurs when the user can start to interact with the page; TTI occurs when the user is fully able to interact with the page. See Google's First Interactive And Consistently Interactive if you're interested in the exact calculation for each metric.
Your First CPU Idle score is a comparison of your page's First CPU Idle time and First CPU Idle times for real websites, based on data from the HTTP Archive. For example, sites performing in the ninety-fifth percentile render First CPU Idle in about 3 seconds. If your website's First CPU Idle is 3 seconds, your First CPU Idle score is 95.
This table shows how to interpret your First CPU Idle score:
|First CPU Idle metric|
|Color-coding||First CPU Idle score|
(HTTP Archive percentile)
|Over 6.5||Red (slow)||0–49|
See the Lighthouse performance scoring post to learn how your page's overall performance score is calculated.
See How to improve your TTI score. The strategies for improving First CPU Idle are largely the same as the strategies for improving TTI.
Unless you have a specific reason for focusing on a particular metric, it's usually better to focus on improving your overall Performance score.
Use the Opportunities section of your Lighthouse report to determine which improvements will have the most value for your page. The more significant the opportunity, the greater the effect it will have on your Performance score. For example, the Lighthouse screenshot below shows that eliminating render-blocking resources will yield the biggest improvement:
See the Performance audits landing page to learn how to address the opportunities identified in your Lighthouse report.