We're doing tons of optimizations, trust me. :) Working on compile times is a constant, high-priority request from users. rustc is also a Rust project, and so we want Rust programs to compile faster too. It's just non-trivial. If you've got a suggestion to make rustc compile quickly with little effort, we're all ears!

D, on the other hand, was always designed for fast compilation (If I recall correctly), and so it compiles quickly. It's important to us, but not as important as other things. You can't be the best at every task, so you have to prioritize. For example, take two implementations of D: DMD and LDC. As I understand it, DMD uses its own codegen, so it compiles quickly, but produces slower binaries. LDC, based on LLVM, compiles more slowly, but produces faster binaries, for the exact same code. My understanding is that LDC is still faster to compile than rustc, but these are the kinds of tradeoffs you have to make.