Build Performance

This guide contains some useful tips for improving build/compilation performance.

---

General

The following best practices should help, whether you're running build scripts in development or production.

Stay Up to Date

Use the latest webpack version. We are always making performance improvements. The latest recommended version of webpack is:

Staying up-to-date with Node.js can also help with performance. On top of this, keeping your package manager (e.g. npm or yarn) up-to-date can also help. Newer versions create more efficient module trees and increase resolving speed.

Loaders

Apply loaders to the minimal number of modules necessary. Instead of:

module.exports = {
  //...
  module: {
    rules: [
      {
        test: /\.js$/,
        loader: 'babel-loader',
      },
    ],
  },
};

Use the include field to only apply the loader modules that actually need to be transformed by it:

const path = require('path');

module.exports = {
  //...
  module: {
    rules: [
      {
        test: /\.js$/,
        include: path.resolve(__dirname, 'src'),
        loader: 'babel-loader',
      },
    ],
  },
};

Bootstrap

Each additional loader/plugin has a bootup time. Try to use as few tools as possible.

Resolving

The following steps can increase resolving speed:

• Minimize the number of items in resolve.modules, resolve.extensions, resolve.mainFiles, resolve.descriptionFiles, as they increase the number of filesystem calls.
• Set resolve.symlinks: false if you don't use symlinks (e.g. npm link or yarn link).
• Set resolve.cacheWithContext: false if you use custom resolving plugins, that are not context specific.

Dlls

Use the DllPlugin to move code that is changed less often into a separate compilation. This will improve the application's compilation speed, although it does increase complexity of the build process.

Smaller = Faster

Decrease the total size of the compilation to increase build performance. Try to keep chunks small.

• Use fewer/smaller libraries.
• Use the SplitChunksPlugin in Multi-Page Applications.
• Use the SplitChunksPlugin in async mode in Multi-Page Applications.
• Remove unused code.
• Only compile the part of the code you are currently developing on.

Worker Pool

The thread-loader can be used to offload expensive loaders to a worker pool.

Persistent cache

Use cache option in webpack configuration. Clear cache directory on "postinstall" in package.json.

Custom plugins/loaders

Profile them to not introduce a performance problem here.

Progress plugin

It is possible to shorten build times by removing ProgressPlugin from webpack's configuration. Keep in mind, ProgressPlugin might not provide as much value for fast builds as well, so make sure you are leveraging the benefits of using it.

---

Development

The following steps are especially useful in development.

Incremental Builds

Use webpack's watch mode. Don't use other tools to watch your files and invoke webpack. The built-in watch mode will keep track of timestamps and passes this information to the compilation for cache invalidation.

In some setups, watching falls back to polling mode. With many watched files, this can cause a lot of CPU load. In these cases, you can increase the polling interval with watchOptions.poll.

Compile in Memory

The following utilities improve performance by compiling and serving assets in memory rather than writing to disk:

• webpack-dev-server
• webpack-hot-middleware
• webpack-dev-middleware

stats.toJson speed

Webpack 4 outputs a large amount of data with its stats.toJson() by default. Avoid retrieving portions of the stats object unless necessary in the incremental step. webpack-dev-server after v3.1.3 contained a substantial performance fix to minimize the amount of data retrieved from the stats object per incremental build step.

Devtool

Be aware of the performance differences between the different devtool settings.

• "eval" has the best performance, but doesn't assist you for transpiled code.
• The cheap-source-map variants are more performant if you can live with the slightly worse mapping quality.
• Use a eval-source-map variant for incremental builds.

Avoid Production Specific Tooling

Certain utilities, plugins, and loaders only make sense when building for production. For example, it usually doesn't make sense to minify and mangle your code with the TerserPlugin while in development. These tools should typically be excluded in development:

• TerserPlugin
• [fullhash]/[chunkhash]/[contenthash]
• AggressiveSplittingPlugin
• AggressiveMergingPlugin
• ModuleConcatenationPlugin

Minimal Entry Chunk

Webpack only emits updated chunks to the filesystem. For some configuration options, (HMR, [name]/[chunkhash]/[contenthash] in output.chunkFilename, [fullhash]) the entry chunk is invalidated in addition to the changed chunks.

Make sure the entry chunk is cheap to emit by keeping it small. The following configuration creates an additional chunk for the runtime code, so it's cheap to generate:

module.exports = {
  // ...
  optimization: {
    runtimeChunk: true,
  },
};

Avoid Extra Optimization Steps

Webpack does extra algorithmic work to optimize the output for size and load performance. These optimizations are performant for smaller codebases, but can be costly in larger ones:

module.exports = {
  // ...
  optimization: {
    removeAvailableModules: false,
    removeEmptyChunks: false,
    splitChunks: false,
  },
};

Output Without Path Info

Webpack has the ability to generate path info in the output bundle. However, this puts garbage collection pressure on projects that bundle thousands of modules. Turn this off in the options.output.pathinfo setting:

module.exports = {
  // ...
  output: {
    pathinfo: false,
  },
};

Node.js Versions 8.9.10-9.11.1

There was a performance regression in Node.js versions 8.9.10 - 9.11.1 in the ES2015 Map and Set implementations. Webpack uses those data structures liberally, so this regression affects compile times.

Earlier and later Node.js versions are not affected.

TypeScript Loader

To improve the build time when using ts-loader, use the transpileOnly loader option. On its own, this option turns off type checking. To gain type checking again, use the ForkTsCheckerWebpackPlugin. This speeds up TypeScript type checking and ESLint linting by moving each to a separate process.

module.exports = {
  // ...
  test: /\.tsx?$/,
  use: [
    {
      loader: 'ts-loader',
      options: {
        transpileOnly: true,
      },
    },
  ],
};

---

Production

The following steps are especially useful in production.

Source Maps

Source maps are really expensive. Do you really need them?

---

Specific Tooling Issues

The following tools have certain problems that can degrade build performance:

Babel

• Minimize the number of preset/plugins

TypeScript

• Use the fork-ts-checker-webpack-plugin for typechecking in a separate process.
• Configure loaders to skip typechecking.
• Use the ts-loader in happyPackMode: true / transpileOnly: true.

Sass

• node-sass has a bug which blocks threads from the Node.js thread pool. When using it with the thread-loader set workerParallelJobs: 2.