We're loosing a lot of performance because of an extra IPC roundtrip when going from C++ through C to Rust.

Let's use the Arrow C ffi instead!
https://arrow.apache.org/docs/format/CDataInterface.html#structure-definitions

Further improvement from there is to not send the schema every time, but that's an extra step