Some things compile decently faster (~ 10%), some things compile a little faster or slower (~ +/- 3%), some things have a bigger perf hit but not as many as had a bigger perf gain (~ -10%).
So it's faster on average but the data is muddy enough that you probably wouldn't stick it front and center on your release notes.
From the graph, it points out this commit. And from the description, I think it's exactly that commit and not just one close to it. I heard it was a big improvement.
I'm talking about the change on the 17th of October, only a couple of days ago, not the one from a month ago which I know (and mentioned) is the new pass manager. The change doesn't show up in all of the graphs, but for the cases where it does show, it's a similar size improvement to the new pass manager.
Maybe it was enabling PGO rather than any code change, I've heard it mentioned that happened recently.
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.
