Running the build with -ftime-trace almost always has shown in my experiments that the bottlenecks are either in exploding number of template instatiations or more often than not in linking time.
Years ago the GNAT Ada compiler authors intentionally avoided implementing precompiled headers. They instead focused on very fast lexical analysis (using a small handwritten lexer optimized for lower case letters because that was the usual case). By careful optimization of a small part of the compiler they made the recompilation of headers faster than loading a precompiled header!! More info: https://dwheeler.com/essays/make-it-simple-dewar.html
Maybe the Rust team needs to add timers to their test suite. This kind of big regression in compilation time should have been caught by the test suite.
I assure you it's not an afterthought. We track performance changes every week, going as far as reverting highly desired features to avoid regressions. The only reason we accept (small) regressions is for correctness fixes. This regression was only partly exercised by our perf test suite. That's how we grow it: see bad behaviour in the wild, add to the suite.
The source of rust compilation slowness usually boil down to excercising a part of the language that has O(n²) behaviour, like match arm evaluation, trait bound evaluation, or huge expansion due to monomorphization or macro expansion, and the resulting huge linking times from that, or the bottleneck that proc-macros introduce (they get compiled before they can get executed, only relevant on fresh compiles).
The regression was (partly) fixed by adding caching: https://github.com/rust-lang/rust/pull/90423/files and if I recall correctly was introduced by a feature change that fixed a bunch of incorrectly rejected deeply nested trait bounds.