Although monomorphization takes a significant chunk of a typical Rust compilation, it pales in comparison to LLVM codegen and execution of procedural macros. So although it's academically worthy of note, optimization-wise it's not where the focus should be. Personally, I'd like to see someday a Rust compiler which is not based on LLVM. Both Zig and Jai compile much faster when using their non-LLVM backends than when using LLVM, so Rust is not the odd one here either.
> Although monomorphization takes a significant chunk of a typical Rust compilation, it pales in comparison to LLVM codegen
It's not the monomorphization itself that takes a long time, it's the LLVM codegen due to all the extra code being generated from duplicated function implementations. I've heard macros slow for largely the same reason, it's not the macro execution itself that's slow, it's compiling all the generated code.