First CPU Idle

Caution: First CPU Idle is deprecated in Lighthouse 6.0. While some have found that First CPU Idle offers a more meaningful measurement than Time To Interactive, the difference isn't significant enough to justify maintaining two similar metrics. Moving forward, consider using 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.

What First CPU Idle measures

First CPU Idle measures how long it takes a page to become minimally interactive. A page is considered minimally interactive when:

  • Most—but not necessarily all—UI elements on the screen are interactive, and
  • The page responds, on average, to most user input in a reasonable amount of time.
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.

How Lighthouse determines your First CPU Idle score

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
(in seconds)
Color-coding First CPU Idle score
(HTTP Archive percentile)
0–4.7 Green (fast) 75–100
4.8–6.5 Orange (moderate) 50–74
Over 6.5 Red (slow) 0–49

How to improve your overall Performance score

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.