The preprocessor can be expected to read files and scan to the corresponding #endif very fast. Reading from cache is also very fast. C++ compilation bogs down

The preprocessor can be expected to read files and scan to the corresponding #endif very fast. Reading from cache is also very fast. C++ compilation bogs down when templates are involved. You want to split those (in a way which makes sense to the human reader), so that only the ones needed for a given compilation unit are instantiated. Further the link stage, which is difficult to parallize, is a bottleneck.