It's really hard to write benchmarks of an RPC system that mean much, but the overhead is really just in serialization. We have services that handle tens of thousands of requests per second on Twirp in one process.

Serialization/deserialization of a typical protobuf struct takes a microsecond or two, but it generates some garbage, so GC ends up slowing you down if you try to go really crazy and push past 100k req/s in one process with non-trivial message structures.