[niftich]

> Is it just a perf thing?

Mostly. I can't speak to protobuf's serialization format personally, but similar binary serialization like Thrift's TCompactProtocol or TDenseProtocol outperform JSON and you get schemas for "free*" (as in, in your producer and consumer's glue code by virtue of codegen, and not as an afterthought).

The IDL and codegen is a big part of gRPC and Thrift. The rest is opinionatedness and less cognitive effort -- the format is non-human-readable anyway so less propensity for bikeshedding about cosmetic stuff in JSON.