Comment by knuckleheads

Result:

Headstart on codex-rs (16 cores, clean builds, median of 3 alternating off/on pairs):

                          off       on     saved
  cargo check           383.8s   277.1s    28%
  cargo build           530.7s   330.8s    38%
  check, -Zthreads=8    220.0s   190.5s    13%
  build, -Zthreads=8    317.3s   249.2s    21%

Not helpful for compilation yet, but it's something :)