node-sass-json-importer: Slow incremental webpack build times

I understand that this might not be the right place to report this issue, but I thought I’d post it here first to see if anyone else has more context.

We have a fairly large React app where we use webpack to compile our application.js bundle. Stylesheets are resolved using sass-loader, and components import their own stylesheets, like this:

const styles = require('./index.scss');

const Component = React.createClass({
  render() {
    return <div className={styles.
  }
});

To share configuration between the components and sass files (we use scss), we have a shared_config.json file with some key-value settings. In the past, we’ve used a manual script to generate a shared_config.scss file that we then use in our scss files. Yesterday, (thanks to https://github.com/Updater/node-sass-json-importer/pull/13) we switched to directly importing the json file, through the help of node-sass-json-import. That worked out well, the only problem is that our incremental build times for our webpack bundle went up from ~2.5 seconds to ~12 seconds. An incremental build happens any time a .scss or .js* file is saved. I’ve spent some time to bisect our git history to make sure that it was in fact adding this to our webpack.config.js that introduced the performance regression:

  sassLoader: {
    importer: require('node-sass-json-importer'),
    includePaths: [
      path.resolve('./app/assets/stylesheets'),
    ],
  }

Any ideas what could be the problem? I’m suspecting that node-sass/libsass isn’t being smart enough with custom importers (after all, node-sass warns about this being an experimental feature).