Karma Webpack preprocessor is slow

When running

npm run karma -- karma.watch.conf.js --auto-watch --no-single-run

with

preprocessors: { 'spec-bundle.js': [ 'webpack', 'sourcemap' ] },

webpack preprocessor is working for 6-8 minutes, constantly consuming 13% CPU. Simple Can I optimize this somehow?

Simple npm start

webpack-dev-server --config webpack.conf.js --progress --profile --colors

is ready after about 1 minute.