I read through this article on chromium build time analysis
Its a good insight into what header inclusion can do to build times. We usually count lines of code
but it really should be the lines of code that compiler has to read through when considering build impact
A bit of analysis shows that it makes sense that there is no correlation between source-file length and compile time. The 30,137 compile steps that I tracked consumed a total of 11.6 million lines of source code in the primary source files. However the header files included by these source files added an additional 3.6 billion lines of source code to be processed. That is, the main source files represent just 0.32% of the lines of code being processed.