terser-webpack-plugin: Webpack build hangs with parallel: true

• Operating System: Windows 10 1810, WSL, Ubuntu 18.04
• Node Version: 8.12.0
• NPM Version: 6.4.1
• webpack Version: 4.19.1
• terser-webpack-plugin Version: 1.1.0

Expected Behavior

That the webpack build completes

Actual Behavior

The webpack build hangs forever (or at least 30 minutes which is the longest I have lasted) after outputting

Compilation  starting…

However changing parallel: true to parallel: false in terser options will cause the build to complete in normal time (around 40s)

Code

new TerserPlugin({
        terserOptions: {
          parse: {
            // we want terser to parse ecma 8 code. However, we don't want it
            // to apply any minfication steps that turns valid ecma 5 code
            // into invalid ecma 5 code. This is why the 'compress' and 'output'
            // sections only apply transformations that are ecma 5 safe
            // https://github.com/facebook/create-react-app/pull/4234
            ecma: 8
          },
          compress: {
            ecma: 5,
            warnings: false,
            // Disabled because of an issue with Uglify breaking seemingly valid code:
            // https://github.com/facebook/create-react-app/issues/2376
            // Pending further investigation:
            // https://github.com/mishoo/UglifyJS2/issues/2011
            comparisons: false,
            // Disabled because of an issue with Terser breaking valid code:
            // https://github.com/facebook/create-react-app/issues/5250
            // Pending futher investigation:
            // https://github.com/terser-js/terser/issues/120
            inline: 2
          },
          mangle: {
            safari10: true
          },
          output: {
            ecma: 5,
            comments: false,
            // Turned on because emoji and regex is not minified properly using default
            // https://github.com/facebook/create-react-app/issues/2488
            ascii_only: true
          }
        },
        // Use multi-process parallel running to improve the build speed
        // Default number of concurrent runs: os.cpus().length - 1
        parallel: true,
        // Enable file caching
        cache: true,
        sourceMap: shouldUseSourceMap
      })

Full webpack config: https://github.com/Pajn/tscomp/blob/cra2/config/webpack.config.prod.js

How Do We Reproduce?

I suspect this can be related to WSL. If you have some debugging tips I can probably run them.

For the exact same state as I have:

1. Clone https://github.com/Pajn/tscomp
2. git checkout terser-bug
3. Clone https://github.com/Pajn/RAXA
4. cd RAXA/packages/web
5. yarn
6. yarn add --dev path/to/cloned/tscomp/repo
7. yarn build, or for more information NODE_ENV=production webpack --config node_modules/tscomp/config/webpack.config.prod.js --verbose --info-verbosity verbose --display verbose --profile