The oral exams scale better than one might expect, at least if you're not just doing multiple choice exams and you have to actually grade the exam. The time spent on grading the exam could have also been spent on an oral exam... For bachelor level it's usually 15-20min (usually quicker for the well prepared students), and for master courses it is 30min.

Usually you know after a few minutes if it's going to be a fail, and then otherwise you only need to figure out where on the passing scale the student ends up.