I wouldn't call houseki no kuni slow in the same way. It's more... Artistic. It deals with artistic image based allegory and poetically displaying things like the state of mind of characters through a sort of body horror that occurs. The characters are gems, thus their bodies can be fragile and shatter in a way that you can't do with normal bodies. This opens up the act of someone literally shattering to represent their mental state shattering, that kind of thing. It's absolutely beautiful and was the first fully 3d anime to actually use the 3d in a way that contributes something to the medium that you absolutely couldn't do without the 3d. It's main character's problem is also very very trans with a strong ship of theseus thing going on as she goes through changes, and the community of gems is a commune. All in all my favourite show in a very very long time, it's the kind of show you can analyse at a very very deep artistic level, you can watch it a dozen times and pick up new things in it to analyse every time, new meanings to everything. It's as close to being "art" as it can be.

I liken the two mainly because the way houseki no kuni is representing a trans experience is somewhat similar to the way this show is representing a neurodiverse experience, although this show is far far less art-ey in the way it goes about it.