It's not just a question of engineering resources; some problems are intrinsically hard. There may well be things (like floats in css) in rust that make fast compilation hard if not impossible - depending on what exactly you want to achieve.

After all, the point is to generate efficient code, so a solution that simply disregards most optimizations isn't an option; and most of the type system is likely unavoidable and/or critical to the optimizer.

By contrast, asm.js is fairly simple: All the heavy lifting of compiling has mostly been done - the remaining JS was chosen so that it fairly straightforwardly represents assembler instructions. Decisions about types and what that means for semantics (and safety) have been done, inlining, code rearranging etc. - that's all largely done. In a way, asm.js is assembler with a weird syntax and environment. I really doubt you'd ever get something like Rust (or really most any other statically compiled language) to ever compile as fast, given comparable resources.