I'm imaging more along the lines of serializing editor state with each key press, encoding it, sending it to another process, having that process unserialize it, lex it, encode all of the cascading token changes and information about those tokens back into JSON, sending it back to the editor process which then has to decode the JSON and apply all of the token information to the in-memory representation as real overhead.

It's great that Rust will decode and encode the JSON quickly (I've been keeping an eye on Rust's MSVC progress before I consider using it for a real-world project), but the project I work on cares about optimizations on the level of in-memory data layout and memory allocations. Moving syntax highlighting into a pluggable system would be a huge performance regression.