[int_19h]

> Ancient wisdom about C used to be that more than 50% of the compilation time is spent on preprocessing.

Ancient wisdom was that more than 50% of the time is spent compiling the headers, after they become a part of your translation unit after preprocessing. I don't see why preprocessing itself would ever be singled out, given that it's comparatively much simpler than actual compilation.

[josefx]

Opening and reading all the included files could be costly. Also it is "ancient" wisdom so it might predate compilers that could detect the include guard pattern and had to repeatedly preprocess the same files. There is an ancient "Notes on programming" article by Rob Pike that comes up every now and then with a paragraph against include guards for that outdated reason.