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JavaScript Bootup Time Is Too High


This audit measures the total impact of JavaScript on your page's load performance. JavaScript can slow down your page in many 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.

See JavaScript Start-Up Optimization by Addy Osmani for more data.


Transmission size is critical for low-end networks. Parse time is important for CPU-bound devices. Keeping these low matters. --- JavaScript Start-Up Optimization
  • Only send the code that your users need.
  • Minify your code.
  • Compress your code.
  • Remove unused code.
  • Cache your code to reduce network trips.

See JavaScript Start-Up Optimization by Addy Osmani for more guidance.

More information

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