As the readme notes, it's completely unsafe if you perform any mutation operation.
As to the performance hit, I'd expect almost none when not in use for long-running processes: it spawns a thread, and that thread will then wait on a socket accept, no connection, no resources spent. You'll still be paying in memory for the server thread, but that's it (the server thread only creates two objects, an rlcompleter.Completer instance and a function).
As the readme notes, it's completely unsafe if you perform any mutation operation.
As to the performance hit, I'd expect almost none when not in use for long-running processes: it spawns a thread, and that thread will then wait on a socket accept, no connection, no resources spent. You'll still be paying in memory for the server thread, but that's it (the server thread only creates two objects, an rlcompleter.Completer instance and a function).