When Zen monks are completing a task, why are they doing it very quickly?

I shoot in the blue, vote me down, if you like.

Could be, that this is not so much a zen but more a Japanese thing? Something in there cultural tradition. For sure, there are strong cross-influences between Japanese culture and zen and its hard to separate them. But the overall "hurry" and "no time" is something, I associate with Japan in general.

Any native here to clarify?

When beginning walking meditation, one should go very very very slow, noticing each gradation of each step: lifting, pushing, dropping, placing. Repeat for each leg.

This going slow is common practice instruction in all Buddhist walking meditation manuals if one will do some research.

Eventually, once one has dhyana, one's mind is fully absorbed into the meditation and one can walk much faster without having one's attention falter. Eventually, one can even run while in dhyana!

They are moving fast because they don't think. Acting on your non-conceptual (pre-conceptual) knowledge/wisdom/intuition is one of the most important Zen practices. So important that sometimes the word "Zen" itself is taken to mean such action. When action is mediated by thought it has an element of calculation in it, an element of doubt, an element of worrying about personal profit. Wu-wei -- literally "no action" -- actually stands for action that is not based on dualistic mind, not targeted at attaining (an egoistic) goal, it is action without an actor and therefore fast.

"Don't spare yourself" seems to be the key motivating factor in this Zen monastery. By working hard and not wasting a moment even in lying down they are training their subconscious to become infected with the enthusiasm. If they keep at this until it becomes a habit, soon the rest of their being obeys. Until then they need the keisaku (stick) wielding Jikijitsu.

Theravada retreats can set a gruelling pace too - the make you wake up at dawn and meditate all day on one meal a day. The teacher can get quite worked up if a student misses a sitting. Of course, some Zendos like here can take it to the extreme.

This harsh self denial isn't exclusively Zen or Buddhist, here's Sadhguru Jaggi Vasudev, the Hindu mystic warning against mollycoddling the self in times of illness (Youtube video).

Armies do this too; in boot camps new recruits wake up at the crack of dawn, and make the bed neatly in precise moves, and completed in 20 seconds. Next, a morning shower in exactly 2 minutes and so on. Initially they of course only act like they are self disciplined to avoid punishment, but soon acting sharp and smart becomes a life long habit even without the drill sergeant.

Thich Nhat Hanh, the Vietnamese Zen master disagrees with the use of the keisaku and likes to joke that no one here beats people with a stick unlike in other Zen monasteries. Motivation and encouragement is given through love to oneself and one's inner subconscious. As the other answerer noted, Plum Village has an air of deliberate compassion. There's no hurry, but there's no laziness either. In my opinion, we could all use more places like Plum Village, but it is my experience that PV's gentle way is the exception not the rule. I haven't practiced with Ajahn Brahm, but from his talks he hints that he is very gentle with his students, unlike other Theravada retreats.

Edit - running helps purge the lactic acid out of legs that are almost always aching after long bouts of meditation. Not sure how I forgot to mention that fairly obvious point in my original answer!

Everything that goes on at a Zen monastery is carefully considered. From the time you wake up to the moment your eyes close for the night, everything is done to maximize the potential for awakening. With that said, it is extremely difficult to pick out how one aspect of Zen training contributes to awakening. The entire training has to be considered in gestalt.

There are a couple of things at play here. The main one to consider is that one of the most salient difficulties that practitioners face at sesshin is exhaustion. In our sangha, we wake up at 3:30AM. To quote Chris Griffin - "3:30AM?? I didn't even know there was a 3:30AM! What else haven't you told me?!" That's about four to five hours of sleep - tops. Unless you've actually tried getting up that early to recite sutras for an hour and then sat for two hours before getting a break, you really can't wrap your head around how tiring that is. By 6:00AM, you're done - despite the fact that you have a whole day of sitting ahead of you. Those periods of intense, fast activity really help jar you out of sleepiness. They are like a smack of the keisaku and help you from falling back into your own head.

Zen (and I'm speaking of Rinzai Zen here...which is what I practice) doesn't really put a whole lot of weight behind mindfulness training. All that running and scrubbing and what not are done for a very different purpose than what you might find at a Plum Village or Theravada retreat. I think it's important to view this sort of training for what it is and not try to view it through the lens of other Buddhist practices.