https://sr.ht/~hadronized/kak-tree-sitter/ exists. I have not used it personally but the author has been pretty active and we had multiple discussions around performance, AFAIU it does not rely that much on the shell as most communication with Kakoune can take place directly over fifos.

Similarly https://github.com/kakoune-lsp/kakoune-lsp exists and has been a pretty good showcase of how complex plugins can be implemented in other languages than shell (rust here) with the extension model.

I am of course pretty biased, but I am regularly surprised by how simple/quickly things can be done with Kakoune's shell based model, and I its a part of the design I am pretty happy about in retrospect.