I suspect that there are a number of factors involved. One might be whether any support was available — whether the recipient was bending over a desk, or alternatively touching toes. I can imagine that the former would be much easier, as it would be possible to hold on to the desk, grit one’s teeth and wait until it was over, even if in considerable distress at the time.
A rather different question is whether a kind of numbness sets in as the punishment proceeds. This might have either physiological or psychological causes. I imagine that someone like KK would have views about this.