Hi @wolfgang.rumpf,
I suspect this may have more to do with the system you are running this on, and how many resources you are requesting, rather than QIIME 2. If you are running this on a server or compute cluster, you should discuss with the administrator to make sure you are requesting resources properly. If not, please let us know a little more about your system. q2-feature-classifier is definitely configured to use multiple cores!
I have tried the --p-n-jobs -1 setting to no avail. Resource request and allocation are "correct" (I use this system for many different types of applications), but that doesn't mean that the system and QIIME2 are interacting properly. I'll poke the admin and see if he can help!