If people used precompiled headers (which have been available for about 20 years) that problem wouldn't be there...
However I'm not sure that the parsing is such a bottleneck these days; I think all the complex constructs are a lot more of a time sink (templates etc). That and linking! Takes an hour to link the webkit library with the regular linker :-)
I have no doubt that they can make the parsing stage faster, as they won't be parsing the same headers over and over again. This is often a bottleneck in C++, but much less often in C. (I've seen 10+MB C++ files after preprocessing).