If the perf of gRPC is not good enough, you should file an issue on Github. I'm kind of surprised that gRPC loist by 2x. That's about how much better netperf is.

When you say you used Cap'n'proto, did you mean the RPC and serializer, or just the serializer? I ask because gRPC's serialization is pluggable, and doesn't have a dependency on Protobuf.