A simple question: I've got a cron that executes every 5 minutes, and the job itself also runs for some 5 minutes.
Do all the other cron tasks wait for this job to finish? Or does Magento 2 run crons in parallel?
bin/magento cron:run command runs synchronously. It works with the event/observer mechanism of the Magento framework, cron jobs are observers for the "default" event in the "crontab" area.
That means, all other tasks that would be scheduled for the same time wait until your long running job finishes