The stock protobuf implementation is pretty close to optimal given what it does. Where upb is faster, it is faster by doing less.

For example, I can beat protobuf if you don't write the parsed data into a tree structure (or only write a few fields). I can beat it if your input has unknown fields and you don't care about preserving them. But if what you need is a tree structure that contains 100% of the input data, protobuf is hard to beat speed-wise (though there are still a few tricks, like arena allocation, that can beat it given some usage patterns).