I tested 9 students yesterday for their Kyu ranks, most of them rokkyus, a few in the middle and two brown belts. The brown belts still have to demonstrate their cutting on Sunday the 13th to complete their test.
It took a good hour or so to do the testing but it seemed to go pretty quickly for me. It seems to take forever of waiting and then take a long time to do your own test, but in reality time passes pretty quickly. There's also the nervousness factor. It makes you forget what you already know. I was pretty generous and allowed people to start back from the beginning when they forgot something. I can tell when someone forgot something because they were nervous versus when they truly did not know.
Call it nervousness, performance anxiety, or stage fright, its the same thing: performing under pressure. Tests can be stressful, but the real evidence appears if you can perform the techniques during combat. From the martial perspective, that is what demonstrates if someone really knows what they are doing.
In a way, that also explains why our katas are relatively simple and focused on practical techniques that you will remember during combat, rather than a complex set of actions involving dozens of steps. The additional fact is that swordfighting tends to be much more immediate. A properly executed cut is typically lethal. You simply don't need to keep attacking the target over and over again.
The more complex cuts tend to anticipate block and counterattacks, or aim to test other factors like accuracy, speed, cutting range, dynamic reaction, multiple opponents, etc. The best cut is still just a single cut executed in perfect form.