Incidentally your first question is one that's very amenable to testing. When does the Rust compiler spend most of it's time? Is it at the checking stage?
rustc has a self-profiler that can be used to answer this question [0], as well as a mode that times compiler passes [1].
There's no single reason the Rust compiler is slow, as it depends quite heavily on the code being compiled. For some codebases, LLVM code takes up most of the time; in other codebases (e.g., extremely generic-heavy codebases), it'll be checking-related passes.
[0]: https://github.com/rust-lang/measureme/blob/master/summarize...
Yes, you're technically correct, but also being unreasonable.
It was pretty clear that "zero cost abstractions" refers to the runtime aspect. That is the default meaning, there's nothing to "get back to".