It seems google is claiming their protocol buffers are faster than xml... without any proof.
Consider AsmXml, which can process xml at over 200MB/s on old machines.
The protocol buffers from google also generate wrappers for different languages, and other nice things. But for loading structures into and out of memory, xml can be very fast.
Before claiming things like that, I think proof in the form of benchmarks are needed.
I don't doubt they thought that xml was slower, since many implementations are slower. Maybe xml is slower, but there is no proof yet. Also I'm sure the other nice features of protocol buffers make them perfectly suited for their task.
Url encoding could have been used nicely too.