I don't really get your attitude here. In particular, I'm not disagreeing that JSON parsers in the browser could be currently faster than protocol buffers.

I'm saying that computer science and hardware dictate that protocol buffers are faster for a wide range of reasons. That part's not in question- smaller data encodings have better cache use, and require far fewer dictionary (hash table) lookups at parse time, as well as far length work parsing strings. If you want to argue against my point there I don't know what to say.

If it was a priority to write a blindingly fast protocol buffer parser in JS, it's almost certain than an expert could write a faster one than a similar JSON parser.