What is identified above as a mere commuter annoyance is in fact a
bona fide psychological/physiological effect, so much so that as a
distance runner I was coached on how to overcome it.
The point of running, or at least competitive running, is to
stretch yourself to your sustainable limit and hold it there for the
duration of the race. The best runners--those in top physical
shape--can usually get into a zone (the so-called "runner's high")
where the endorphins are flowing and one's body is drawing energy
out like a well-oiled machine, but for the rest of us scrubs it is
rather uncomfortable: your legs are tired, your lungs aren't getting
enough oxygen, and so forth.
To make things worse, your personal wellspring of energy will ebb
and flow over the course of the race, so there will be distinct times
where you will feel waves of severe tiredness come over
you, and during these periods it is extremely important not to slow
down or reduce your pace. Runners who do slow down find that they
get into a death spiral where the sudden reduction in effort sends a
very strong message deep from the recesses of your brain that IT IS
TIME TO STOP NOW. Then they slow even more, or outright just stop
running even as the rational component of their brain is on the
sidelines screaming you idiot, you are supposed to be running a
race. The more sudden the reduction in effort, the more this
time-to-stop message seems to arrive like a freight train.
Running on the flats you pretty much have control over this if you
can just work mentally on keeping yourself at pace and in the zone,
but if you're running cross-country there are hills and such, and
reaching the top of a hill that lethal energy drop hits you
unavoidably: you've been working extra-hard fighting gravity as you work up the hill, and all of a sudden
you're at the top and it's flat again. The non-runner's intuition
here (you may be thinking) is that this should give you a big burst
of energy, since there's now less effort involved, but it's exactly
the opposite in practice: during that transition every muscle in your
body is screaming for you to stop running.
The technique I learned to help overcome this was perfected in a 5K
cross-country course that ran through a reservation, and went up a
steep hill at around the beginning of the 2nd mile, steep enough that
they had actually installed wooden steps in order to make it runnable.
To keep going at the top of "The Stairs", as every team in the
conference referred to them, was difficult enough that our coach
taught us a trick: there were 75 or so steps, so when we reached the
beginning we would count back from 100 as if we actually had 100 to
go. When you hit the top your brain would be thinking "25...24..."
and it's amazing how much this helped. It's a trick, you know it's a
trick, but somehow it's enough to keep the stop message from ever
arriving (or maybe it just goes by and you don't notice, I've never
really been sure).
So if you translate this into tired, overburdened, out-of-shape
commuters for whom a long set of stairs is tiring, it's entirely
plausible that exactly the same effect is at work, and they're really
slowing down at the top because their body is telling them to.