A new jobs: option has been added to the make command. It defaults to
1. Here's an example that could be used on a machine with 2 cores :

% gsc make jedi jobs: 2

Another way to specify the number of parallel jobs to use is to add
the following line in a $HOME/.jazz/.buildini file :

(jazz.build-jobs 2)

On my Macbook Pro with 2 cores, time of a full jedi rebuild has gone
from 14mn down to 10mn. I estimate that it should go down to maybe 8mn
once the big jazz.ui package has been splitted into clean sub-packages
that can all be built in parallel.

PS: I would be very curious about build time of jedi if anyone here
has access to a machine with 4 or more cores.

Enjoy,

Guillaume