Generating sourcecode asynchronous

improved performance for building the artifacts
Are you sure this is asynchronous? According to http://symfony.com/doc/current/components/process.html#running-processes-asynchronously the start method initiates a asynchronuous execution. 

And we should wait at the end for each of the asynchronous processes to have finished (thus before execution stops) to prevent issues from the application closing before all processes are ready.

Have you tested the performance benefits? The encapsulated code only writes to file, or not?