Reduce JavaScript execution time
Lighthouse reports the time spent executing JavaScript files during page load in the Diagnostics section:
How JavaScript execution slows down performance
JavaScript execution can slow down your performance 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.
Ways to speed up improve JavaScript execution
- Only send the code that your users need (see Reduce JavaScript payloads with code-splitting).
- Minify and compress your code (see Minify and compress network payloads).
- Remove unused code (see Remove unused code).
- Cache your code to reduce network trips (see Apply instant loading with the PRPL pattern).
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