The luncher code is small and straightforward. It is basically a bridge between Webpack and the official npm module.
While in watch mode, the transpilation is extremely quick: Webpack recompile only the file that got modified. But lunching Webpack can take a very long time as it is recompiling everything from scratch.
Webpack watching... Compilation starting at: 19:10:31 Hash: 063a8e90a05b83f43d51 Time: 27427ms
After a fork of the loader and the addition of a file cache, it became much faster.
Webpack watching... Compilation starting at: 19:11:31 Hash: 063a8e90a05b83f43d51 Time: 17760ms
The next step is to send a pull request with this change. Not sure how it will goes, considering that I used sync file I/O for everything :p