In my experience (with Rust programs that take 20mins+) it is all about the monomorphisation of generics. The great majority of the time is spent in LLVM, because Rust hands over many GB of LLVM IR for optimization, having done not very much (edit: in the way of code transformation) other than direct instantiation of the generic parameters.

Rust does have the opportunity to do more, and I suspect they are hoping to do more with MIR, but at the moment they rely on LLVM to optimize out all of the zero-cost abstractions that they introduce.