A lot of the bottleneck is the header structure: the size of the translation units (.c files with headers expanded transitively) is often quadratic with respect to overall codebase size.
If you fix that, then linking can take a long time too. (Not sure on the details, but it's a "global" algorithm to fix up all the pointer offsets)
Incremental builds will be slow if you structure your code in such a way that you often have to touch a header file, and that header file requires recompiling all translation units.
In short, you have to be super careful about structuring your code in C/C++ if you want fast build times, and approximately zero of the industrial codebases I've seen do this. (Well there was one, but it was all written by a single person, which is not surprising.)
That also depends massively on the used libraries and the structure of the code. A 100kloc c file could build superfast. Whereas multiple c++ totalling about 10kloc that include use use lots of Boost stuff can take ages compared to that.