evaluating scancode

Hi @pombredanne,

I've embedded scancode (a really nice tool) in [Graal](https://github.com/chaoss/grimoirelab-graal) and now I'm evaluating scancode against nomos (another popular tool for license analysis) wrt precision and performance.

In a nutshell, the evaluation consists in iterating over the commits of a set of git repositories, for each commit graal performs a checkout and launches scancode/nomos on each file present in the commit, finally the results are persisted on disk.
While nomos is pretty fast (it processed 5 repos of around 3000 commits each in 2 hours), scancode is stilll processing the first repo. I'm wondering if I'm missing some parameters (or if you have some suggestions) to make the analysis faster. Currently I'm using the release 3.0.0 and I launch it with the following params:
https://github.com/chaoss/grimoirelab-graal/blob/master/graal/backends/core/analyzers/scancode.py#L58

Thank you