PGO requires a runtime profile, so I doubt they've enabled that by default :-)
Rust has had LTO for quite a while, and it's normally a source of longer compilation times rather than shorter ones (since LTO in LLVM-world involves mashing all of the bitcode together and (re-)running a lot of expensive analyses to further optimize across translation unit boundaries.
OTOH they've been making continuous improvements to the incremental compilation mode since 1.51/2, so that's probably among the sources of improvements here.
Rust has had LTO for quite a while, and it's normally a source of longer compilation times rather than shorter ones (since LTO in LLVM-world involves mashing all of the bitcode together and (re-)running a lot of expensive analyses to further optimize across translation unit boundaries.
OTOH they've been making continuous improvements to the incremental compilation mode since 1.51/2, so that's probably among the sources of improvements here.