import std; is an order of magnitude faster than using the STL individually, if that's evidence enough for you. It's faster than #include <iostream> alone.

Chuanqi says "The data I have obtained from practice ranges from 25% to 45%, excluding the build time of third-party libraries, including the standard library."[1]

[1]: https://chuanqixu9.github.io/c++/2025/08/14/C++20-Modules.en...

We did see build time improvements from deploying modules at Meta.