I see that there are many comments on full debug builds, but for me the most important difference are incremental build times when making minor changes. In my opinion this is what speeds up the development iterations.

Here are my build times when making a trivial change to a print-statment in a root function, comparing nightly dev vs adding cranelift + mold for rust-analyzer[0] (347_290 LoC) and gleam[1] (76_335 LoC):

    $ time cargo build
    Compiling rust-analyzer v0.0.0 (/home/user/repos/rust-analyzer/crates/rust-analyzer)
    # nightly
    Finished `dev` profile [unoptimized] target(s) in 6.60s
    cargo build  4.18s user 2.51s system 100% cpu 6.650 total
    
    # cranelift+mold
    Finished `dev` profile [unoptimized] target(s) in 2.25s
    cargo build  1.77s user 0.36s system 92% cpu 2.305 total

    Compiling gleam v1.0.0 (/home/user/repos/gleam/compiler-cli)
    # nightly
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.69s
    cargo build --bin gleam  3.02s user 1.74s system 100% cpu 4.743 total

    # cranelift+mold
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.99s
    cargo build --bin gleam  0.71s user 0.20s system 88% cpu 1.033 total

For me this is the most important metric and it shows a huge improvement. If I compare it to Go building Terraform[2] (371_594 LoC) it is looking promising. This is a bit unfair since it is the release build for Go and this is really nice in the CI/CD. Love Go compilation times and I thought it would be nice to compare with another language to show the huge improvements that Rust has made.

  $ time go build
  go build  3.62s user 0.76s system 171% cpu 2.545 total

I was looking forward to parallel front-end[3], but I have not seen any improvement for these small changes.

[0] https://github.com/rust-lang/rust-analyzer

[1] https://github.com/gleam-lang/gleam

[2] https://github.com/hashicorp/terraform

[3] https://blog.rust-lang.org/2023/11/09/parallel-rustc.html

*edit: code-comments & links + making it easier to see the differences