What does "It slows the juices down something marvelous" mean?

This is an expression of feelings and emotions from the author.
This has nothing to do with drinking some liquid, however, it has to do with relaxation, and literally it could be taken to mean that the chemistry in the author's brain ("juices") has alternated to a slow, calm and relaxing state, being in the churchyard cemetery, and thus makes the author feel wondrous and "something marvelous."
To put it another way, this visit slows down his pace of life.

Thank you.
"This visit slows down his pace of life" is easy to understand.

But "It slows the juices down something marvelous" is hard to understand grammatically.
Does it mean "it slows down the running speed of the juices in my brain and changes them into something marvelous"?

Don't take my brain reference too literally.

Take it in two pieces and it'll be easier to digest it:

"It slows the juices down something marvelous."

"It slows the juices..."
From the first sentence ("I go back a churchyard cemetery on the side of an Appalachian hill in northern Virginia to call on family elders"), we have a context to take the word juice to mean essence or the intrinsic nature of the author.

"...down something marvelous."
The word "down" is used as an adjective, placed after the noun "juices," to mean directed or moving toward somewhere or someplace below (which could be taken in a literal sense to mean the dead below him who are connected to him, as well as in a metaphorical sense, that his pace of energy is lowered), to the fond memories of the past which is relatable to an experience of "something marvelous."

Right. Let's not get too literal about the juices.

By the way, the setting is Appalachia. It it were a country instead of a region in the US, it might charitably be described as an "emerging nation". That is probably why the writer said "marvelous" instead of "marvelously" It sounds more like the way we expect mountain people to talk.