TCP roundtrip overhead over localhost costs approximately nothing.

But running the LSP itself is slow. It's not instant, and you can't run it in a blocking way for each keypress.

A function call costs nanoseconds. You can sprinkle them literally everywhere. A JSON-formatted TCP round trip to localhost costs, I'd guess conservatively, at least ten microseconds, 1000x slower.

OK. Here's my simple implementation of an LSP:

> func doLspStuff() { sleep(60000); }

Now sprinkle them after each keypress. Do you see the issue now?