The third part of the interview is designed to see how she handles herself under pressure in an unfamiliar situation. He makes it clear that regardless of the demands placed on her, failing to complete the task is not an option.
At several points she finds herself barely able to keep pace with him, having never experienced a boss capable of working at this level for such an extended period of time.
Midway through the exercise he informs her that he’s holding back, since this is just an interview. He explains that if she manages to get the job, she’ll need to perform daily under much more rigorous parameters.