I don't know about 100K, but for my 5667 line project, a recent nightly build takes 8 seconds to build on my 2012 macbook pro (4 core 2.3G i7). That is with up-to-date external crates that aren't being rebuilt.

Typecheck happens early, with borrow-checking following soon after, so you get feedback on errors fast, and thus only end up waiting when the compiler is happy with your code. Still, it may be an issue for people who like to do TDD or need to do a lot of run-time verification.

For a big project you'd probably need to split it up into multiple crates to "emulate" incremental compilation, at least until it is supported by the build system.

That exact property is why cargo check works.

You would definitely split that project up into multiple crates at the moment. 100KLOC would take an eternity to compile. My guess would be 15 to 45 minutes for a release build, depending on things like how much you were using generics.