C parsing is rather slow due to the preprocessor (also: huuuge include files) and due to context-dependent syntax (need symbol table). And compiling itself need not be slow, but the popular compilers are not exactly super fast even with -O0. LLVM in particular has a reputation of being somewhat bloated. And finally, due to common practice of using a standard object file format, which is slow to link (at least with common tools), the whole build experience is not exactly blazingly fast.

With clang/gcc/msvc, I think I'm more in the ballpark of 3K-30K lines/sec (-O0, 100-1000 lines/file, not counting basic std* includes).