Real-world usage of Clang modules (by Google) and preliminary/synthetic benchmarks of C++20 modules in GCC both show about 3x speedup. This might not be "much" for everyone, buy I will take it.

> benchmarks of C++20 modules in GCC both show about 3x speedup

compared to what? The worst possible case? Code in weakly coupled modules? Using PIMPL idiom? Unity builds? Using ccache in incremental builds?

Especially ccache is a relevant comparison, because what to a developer is of far more interest than a full build is an incremental build with a few files changed. Using modules requires to scan the source code in order to determine dependencies, so it can be slower than an incremental build using other tools. And in addition to this, C++ compilation with include headers is an embarrassingly parallel problem which means speed advantages from using multi-core systems will probably very relevant in the, say, ten years time in which C++ modules will be somewhat standardized in practice.

Not to say they can't help, but in any other large software project one would need to show that something is useful before adding a big change.