True this! Using a C++ compiler infrastructure as the backend is fantastic for compiled code speed, but it’s of no help of compiling speed, lol.
They are working on cranelift still though, basically a fast JIT for running debug code near instantly, not as fast but it makes for a fast turnaround time. Then the final Release build will be a full optimization build. 
It can compile dependencies in parallel so it’s not that big of an issue unless you have a really really deep tree or lots of procmacro’s (macro-by-example’s are more fine) helper libraries like syn. There’s a big push to optimize a lot of these paths and remove the noise from the ecosystem, so it is getting better here too. ^.^