I used to agree with this, but having spent a while in a very good C++ project (by C++ standards), even if we try to always forward declare and not include any unnecessary headers in headers, we still have a clean debug build time of 30-40 minutes. Even changing a single line in a cpp file, will still take around 30-40 seconds at best with incremental builds. I think long compile times are in the eye of the beholder, but often it feels like people have just gotten used to unnecessarily long compile times because they've forgotten how fast computers are and how fast a compiler should be for most of the work except for maybe some of the more complex optimizations in -O3. It kills productivity, and should really be a much larger focus than it sometimes seems to be.

I'm yet to see a C++ project that compiles as fast as a C one.

"Bad use of header files" is the fault of C++, not the developers. Until Stroustrup fixes the header processing behavior (it's 2019 already...) C++ will not become a sane language to use for new development.

As soon as you include C++ stdlib headers (even only for the stdlib features you're actually using), you have tens- to hundreds-of-thousands of lines of complex template code in front of your own code, and this in each compilation unit. In the end your own code is maybe one percent of all the code the C++ compiler needs to work through each time something is compiled (depending on how many source files you have, this is also why unity builds - merging all source files into one - are so much faster in C++).

In comparison, CRT headers (when compiled as C, not as C++, as in that case additional C++ bloat is added) are usually a few dozen to a few hundred lines of simple declaration code. Compared to C++, it's very hard to make a C project compile slowly.