I don't know if this is a fair comparison, as 2GB/s of protobuf will parse a lot more information than 2GB/s of JSON will, since protobuf is a much more space-efficient way to encode your data.
See https://news.ycombinator.com/item?id=26934063 , we may get a fairly sound comparison. Though I imagine the comparison varies a lot depending on the data. As another comment mentioned, a message with a lot of large strings would lean heavily towards protobufs. And in that case, the size wouldn't be much different.
I would just measure the size of the resulting data structures. In a language like C/C++ it could be as simple as memory usage before your parsing function and after
