Converting a large mathematical software package written in C++ to C++20 modules

I would like to see a comparison between modules and precompiled headers. I have a suspicion that using precompiled headers could provide the same build time gains with much less work.

From my experience, compile times ain't an issue if you pay a little attention. Precompiled header, thoughtful forward declarations, and not abusing templates get you a long way.

We are commonly working with games that come with a custom engine and tooling. Compiling everything from scratch (around 1M lines of modern C++ code) takes about 30-40 seconds on my desktop. Rebuilding 1 source file + linking comes in typically under 2 seconds (w/o LTO). We might get this even lower by introducing unity builds, but there's no need for that right now.

40 seconds for 1M lines seems super fast, do you have a fast computer and/or did you spend a lot of time optimizing the compilation pipeline ?