Performance is relative. gRPC is plenty fast enough for my use case, and for that matter, almost all client/server use cases that work across the Internet. If a Javascript web client against a REST backend is fast enough latency-wise, then a local gRPC connection on a single PC is gonna feel like greased lightning. Of course, there will be a few scenarios where tight coupling of client/server are required for good enough performance, but they are few and far between.