First CPU Idle and Time to Interactive (TTI) help to pinpoint the time at which a page will respond in an acceptable amount of time.
Why is monitoring Time to Interactive important?
How are these metrics calculated?
Calibre works backwards from the time of document completion until it finds a point at which the browser is at rest. If a user were to interact with the page, there would be notable delay.
Calibre tracks the a number of interactivity metrics: First CPU Idle and Time to Interactive. These are great candidates for improving the user experience.