I'm going to be frank. There really isn't a story. We get a set up: a young man voluntaries for a sensory deprivation experiment and that's basically it. We see his experience during the experiment and right after. For some this might be a problem, but for others who can appreciate thematic one shots this should not pose a problem. I personally believe that this is the best way to approach a one shot since plot and character development cannot really be fleshed out in a one shot, while exploring a single theme is perfect for a short work.
The art is retro, which is not
surprising given the date this was publish: 1985. I personally really like the retro shoujo art, so I really liked the art in this one shot.
The main point of this one shot is exploring the human psyche. While we know very little about the main character outside the unusual circumstances he is placed in, that is ok because this is not about the character himself, but rather the character as a representation of the human mind. He is the human psyche and this story is dealing with our human reactions to the world and to ourselves. It uses this character placed in such an unusual setting to explore one of the most problematic issues in epistemology (the study of human knowledge, especially its limits). That is, the problem of knowledge of reality. And just as the problem still remains unsolved (and probably will forever), so too does the story end without solving the character's problem. When the character is understood in this way, the characterization is well done.
I would say this one shot isn't for everyone. If someone is looking for something entertaining to read, Slow Down is not it. Those looking for something strange and maybe even thought provoking (depending on one's exposure to philosophy), Slow Down is for you. The enjoyment of this one shot is directly tied to the willingness of exploring one's mind along with the main character. Since everything this manga brought to the table wasn't new to me and I quite enjoy a more artistic representative of traditional problems in philosophy, I really liked this one shot. But I will stress again that whether you like this or not highly depends on what you want out of it.