This article analyzes how C compiles multiple libraries and long story short, it provides a genius solution for very quickly improving compile times: https://wiki.alopex.li/LetsBeRealAboutDependencies
Pre-compile the code and store pre-compiled versions on https://crates.io/
Now instead of downloading 100 libraries and compiling every one when you compile a dependency, you instead simply download and run using the pre-built library as built at the moment it was uploaded to crates.io In fact, I believe that for a given chunk of code, typically the exe for just that piece of code(excluding dependencies) is smaller than the source code. So faster downloads too and not that much extra to store on crates.io
Of course, you've want to allow the source code to be downloaded too, that way you can step through it. But how often do you step through the source code of dependencies of dependencies you are using?
We're probably talking at least 10x faster builds?