This is not only not true, but not possible. Many kind sof defintiions in .h files may not be repeated without error, like: struct foo { int bar; }; or t

This is not only not true, but not possible. Many kind sof defintiions in .h files may not be repeated without error, like: struct foo { int bar; }; or typedef int xyzzy_t; This is why you have include guards: #ifndef FOO_H_3DF0_755A #define FOO_H_3DF0_755A struct foo { int bar; } #endif GCC optimized the handling of headers with include guards 30 years ago already. If you think most of your compile time is spent in preprocessing, benchmark a clean build with the optimization set to -O0 versus your full optimization like -O2 or whatever you are using. Both builds perform preprocessing; thus the preprocessing time is bounded by the total time spent in the -O0 build: if the actual semantic analysis of tokens and code generation at -O0 were to take next to no time at all, then we would have to attribute the time to tokenizing and preprocessing. But even then, all the additional time observed under -O2 is not tokenizing and preprocessing.