Here are two alternative Python protobuf implementations. They're each about 15x faster than the pure Python implementation from Google.

fast-python-pb - codegen wrapping Google's C++ protobuf implementation. https://github.com/Greplin/fast-python-pb

lwpb - non-codegen using a protobuf implementation in C. (disclaimer: I'm an author) https://github.com/acg/lwpb