Thanks, do you know what jannson is needed for?
It lets emacs do a bunch of JSON processing in C rather than elisp, which is very helpful for a bunch of cases, especially some LSP modules.
Just in case: Language Server Protocol; a semi-standard way for outboard toolsets like specialized compiler suites to communicate with editors (primarily VSCode, but somewhat generalized and so supported in other editors as well) about things like code completion and language-aware editing. The protocol ends up being pretty chatty, often sending a lot of json back and forth, and the pure-elisp method of converting it had a pretty high overhead.
HtH,
~Chad