JavaScript-Bootup-Time

This audit measures the total impact of JavaScript on your page's load performance.

JavaScript can slow down your page in many ways:

  1. Network cost - More bytes equals longer download times.
  2. 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.
  3. 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.
  4. 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.

Recommendations

  1. Only send the code that your users need.
  2. Minify your code.
  3. Compress your code.
  4. Remove unused code.
  5. Cache your code to reduce network trips.

Tools

Use Closure Compiler or UglifyJS

    npm install uglify-js
    uglifyjs [input files] [options]

Video

Improving Load Performance - Chrome DevTools 101

Further information