The Main Thread Execution Timeline, found on the Overview and Third Party tabs for Snapshots and Single Page Tests, helps you identify long tasks that took more than 50ms to execute during page load. Calibre showcases both parent and children tasks that contribute to a long task warning, so you can identify which specific task causes the most issues.
Each task (identified as Task) is accompanied by a long task warning showing the “blocking period” of each script which contributes to Total Blocking Time and is classified as one of:
The Main Thread Execution Timeline showcases all long task warnings and blocking times in the lifespan of the main thread activity. For that reason, the sum of blocking times portrayed will likely be higher than reported Total Blocking Time, which is limited to the period between First Contentful Paint and Time to Interactive.