My most memorable exam experience as a student was an oral exam in thermodynamics or heat transfer.
They gave a randomly selected question to me and I had 30 minutes to prepare a solution (alone in back of classroom). I provide solution which took 10 minutes then 20 minutes was additional theory questions.
For each student, it took the professor 30 minutes. So for 100 students you can do it in 5 hours.
The downside to oral examinations however is you can't really grade curve them.
Which is a beautiful thing, but also not what the faculty wants.
The faculty grades mostly because they want to cull the chaff from the hardest working grad student candidates.
They aren't after pure comprehension, they want hard workers.