> I am interested by the point of view developed by @larsmans, ie that effort should be put in making the vectorizer faster rather than parallel. 

The comment was more that if a user has a complicated analyzer, they should be profiling that, not the vectorizer which is fairly optimized (for a single processor). And that reading input files asynchronous (if `input != 'content'`) to the counting may be a cleaner way to get the same gains.