Use cheap-module-eval-source-map

By sacrificing a small amount of source map accuracy, we make
incremental builds about 20% faster.

https://webpack.js.org/configuration/devtool/#devtool

webpack.config.js

@@ -162,7 +162,7 @@ switch ( process.env.NODE_ENV ) {
 		break;
 
 	default:
-		config.devtool = 'source-map';
+		config.devtool = 'cheap-module-eval-source-map';
 }
 
 module.exports = config;