Why is the Rust compiler so slow?

So there's this guy you may have heard of called Ryan Fleury who makes the RAD debugger for Epic. The whole thing is made with 278k lines of C and is built as a unity build (all the code is included into one file that is compiled as a single translation unit). On a decent windows machine it takes 1.5 seconds to do a clean compile. This seems like a clear case-study that compilation can be incredibly fast and makes me wonder why other languages like Rust and Swift can't just do something similar to achieve similar speeds.

Every claim I've seen about unity builds being fast just never rings true to me. I just downloaded the rad debugger and ran the build script on a 7950x (about as fast as you can get). A debug build took 5s, a release build 34s with either gcc or clang.

Maybe it's a MSVC thing - it does seem to have some multi-threading stuff. In any case raddbg non-clean builds take longer than any of my rust projects.