[gcommer]

There's tons of little details and inefficiencies in elisp that crop up such that easy things like "listen to responses" are hard to get fast. Random example: before native json support, large lsp responses (which can be multiple megabytes from some servers!) would take over a second and hitch the UI just to parse the json response. And that was after a lot of work to get response parsing dominated by json instead of GC time.