Why is my webpack build slow?

For many of us, webpack is the bottleneck between when we write our code and when we see its side-effects; This is the developer critical path that we feel every single save.

The trope of “my webpack build is slow” is well worn into the ethos of modern web development; This does however not lay blame at the feet of any single tool, but rather is a measure of the systemic acceptance and use of webpack for building web applications. To be clear we are not talking about your JS bundle runtime performance but rather the time that it takes to generate that runtime bundle.

The first step to understanding any system is to measure it, and in order to make something faster we must look at where time is being spent. Luckily when it comes to Node.js we have a cadre of tools to help us profile and understand where time is going for any process. Finding said tools however is a journey strangely intervolven with a smattering of abandoned tooling from multiple-attempts by multiple companies trying to enter the enterprise Node space. I am going to skip all of this noise and cut out all of the stackoverflow and gist sleuthing and jump right into your options:

Approach 1: Using the built in webpack plugin profiler (webpack 4+)

webpack --plugin webpack/lib/debug/ProfilingPlugin

Then load events.json into chrome devtools:

Chrome devtools performance waterfall


Approach 2: Using node’s --prof and --prof-process tooling

node --prof node_modules/.bin/webpack-cli --mode development --config webpack.dev.js
node --prof-process isolate-$ID_HERE-v8.log > processed.txt

You can read more about this here: https://nodejs.org/en/docs/guides/simple-profiling/

It is possible to render the v8 log file further to create a flamechart via

npx pflames isolate-$ID_HERE-v8.log

Classic performance flamechart diagram


Approach 3: Using devtools + --inspect

node --inspect-brk node_modules/.bin/webpack-cli --mode development --config webpack.dev.js

Chrome devtools with node.js debugger icon

Then save the CPU profile and then load the CPU profile back into chrome devtools.

Chrome devtools CPU profile Chrome devtools waterfall diagram


In Summary:

What approach is best for you? Well it depends on your needs and the size of your build. My recommendation is to attempt approach one since it provides the most human readable version of your build and only to drop into the other approaches if you are otherwise unable to get approach one working. Special thanks to Paul Irish for reviewing and assisting in the generation of this document.

Sam Saccone @samccone
1 - It's possible for large webpack builds to encounter out of memory errors during recording with the ProfilingPlugin. When this happens you can first attempt to raise the memory ceiling for the node process via the --max-old-space-size= flag. If this still does not work we can fall back to a more involved solution: The webpack profile plugin implementation waits until the compile is complete to flush out to a file https://github.com/webpack/webpack/blob/master/lib/debug/ProfilingPlugin.js#L203, it is however possible to change this implementation to incrementally flush the results out to a file every N duration… This implementation is something that I will leave up to the reader, but this is open source software so PRs are welcome :)