I've just been looking into the parallel running option a bit more and am left with some questions.

• When should you use this option ?
• Does it work equally well with phpcs as well as phpcbf ?
• What things should you take into consideration when determining the amount of parallel processes to use ?
• What can go wrong ? Are there any type of sniffs which would be incompatible with parallel processing ?
• Any thing else which behaves differently if parallel processing is turned on ?

It might be useful to add a section to the Wiki Advanced Usage page about this.