Yes. If Zig takes 100ms to compile, and Rust takes 2s, but you just spent 20 seconds commenting unused vars in your Zig code, then the slow compiler comes out ahead. It's a great example of how good UX can be more important than raw performance numbers.
Yes, but you comment out unused variable once per variable (you can leave those underscores in and clear them out before release). The 2s happens every time you compile. The badness of slow compiles is non-linear, too. As you start to get to 30s, the likelihood that you check Facebook, Twitter, or hn goes up. At least when you're underscoring unused vars you're doing something active.
If it gets to 2-5 minutes the likelihood that you stop writing tests and rely on the compiler for bugfreeness goes up - or the likelihood that you create debt by increasing the complexity of your code organization.
It's really not a problem for me. I've worked on probably about 20k LOC of zig. Probably the one time it was an issue was when I wrote some code that wrote zig -- and then it pushed me into realizing that this was a code smell, and a refactoring fixed it.
Compile times of 2-5 minutes can happen in langs that are not rust. You mentioned rust, not me. And anyways the 30s to hn thing is real.