I worked on gRPC Java optimization for a few years. The primary reason it is so fast is because it does hardly anything, and avoids synchronization. However, there is a deeper reason why Java was so much faster: more people were working on making it faster. The external C++ version of gRPC was ignored for years, with people focusing on optimizing the internal version. The other languages (C#, Python, etc.) all had significantly less attention that Java did, so they aren't nearly as polished. It's been amusing to watch how stirred up people get about how fast a "language" is, rather than how much time has been spent optimizing the particular implementation.