Reduce JavaScript execution time

When your JavaScript takes a long time to execute, it slows down your page performance in several ways:

• Network cost

  More bytes equals longer download times.

• Parse and compile cost

  JavaScript gets parsed and compiled on the main thread. When the main thread is busy, the page can't respond to user input.

• Execution cost

  JavaScript is also executed on the main thread. If your page runs a lot of code before it's really needed, that also delays your Time To Interactive, which is one of the key metrics related to how users perceive your page speed.

• Memory cost

  If your JavaScript holds on to a lot of references, it can potentially consume a lot of memory. Pages appear janky or slow when they consume a lot of memory. Memory leaks can cause your page to freeze up completely.

# How the Lighthouse JavaScript execution time audit fails

Lighthouse shows a warning when JavaScript execution takes longer than 2 seconds. The audit fails when execution takes longer than 3.5 seconds:

To help you identify the biggest contributors to execution time, Lighthouse reports the time spent executing, evaluating, and parsing each JavaScript file that your page loads.

# How to speed up JavaScript execution

• Only send the code that your users need by implementing code splitting.
• Minify and compress your code.
• Remove unused code.
• Reduce network trips by caching your code with the PRPL pattern.

For other ways to improve page load, check out the Performance audits landing page.

# Resources

Source code for Reduce JavaScript execution time audit