It wouldn’t surprise me if they could do better if they gave up on doing most of the work programmatically.

One part of me agrees with (both from the paper)

> For example, putting a specific piece of code into the right place in each file (or adding necessary header files, as mentioned in Section 5.2) might take 20-30 seconds per file – but doing this for all 1051 files of deal.II then will take approximately a full day of (extremely boring) work. Similarly, individually annotating every class or function we want to export from a module is not feasible for a project of this size, even if from a conceptual perspective it would perhaps be the right thing to do.

and

> Given the size and scope of the library, it is clear that a whole-sale rewrite – or even just substantial modifications to each of its 652 header and 399 implementation files – is not feasible

but another part knows that spending a few days doing such ‘boring’ copy-paste work like that often has unexpected benefits; you get to know the code better and may discover better ways to organize the code.

Maybe, this project is too large for it, as checking that you didn’t mess up things by building the code and running the test suite simply takes too long, but even if it seems to be, isn’t that a good reason to try and get compile times down, so that working on the project becomes more enjoyable?

As per Office team, modules are much faster, especially if you also make use of C++ standard library as module, available since C++23.

See VC++ devblogs and CppCon/C++Now talks from the team.

Pre-compiled headers have only worked well on Windows, and OS/2 back in the day.

For whatever reason UNIX compilers never had a great implementation of it.

With exception of clang header maps, which is anyway one of the first approaches to C++ modules.

Precompiled headers are generally better for system/3rd-party headers. Module are better than PCHs for headers you own, although in some cases you may be better off not using them at all. (I say these because the benefit depends on the frequency with which you need to recompile them, and the relative coupling etc.) Depending on how heavy each one is in your codebase, and how often you modify global build settings, you may have a different experience. And neither is a substitute for keeping headers lightweight and decoupled.

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.