Main Thread Execution Timeline

JavaScript task execution is the source of the biggest and most common performance bottlenecks. Avoiding long-running tasks and blocking the JavaScript main thread is one of the most critical performance strategies to employ.

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.

Each 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:

  • Script Evaluation
  • Script Parsing & Compilation
  • Rendering
  • Style & Layout
  • Parse HTML & CSS

Hovering a task will show the total and blocking duration, and the source of the task as either an identified third party or a JavaScript file.

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