`cargo doc` in large workspace is 10 times slower on nightly

[madsmtm]

Reproduction Steps

• Checkout https://github.com/madsmtm/objc2/tree/815219f441c2f6a67a6787429e4460a43a66c7ff.
• Run cargo +nightly-2025-08-19 doc --workspace --target aarch64-apple-darwin.
• Notice that it finishes in 1-2 minutes.
• Run cargo +nightly-2025-08-20 doc --workspace --target aarch64-apple-darwin
• You don't wanna wait for it to finish.

Logs from my CI on ubuntu-latest:

• On nightly-2025-04-11 (cargo doc takes ~2 mins)
• On nightly-2025-08-24 (cargo doc takes ~24 mins)

Bisection

searched nightlies: from nightly-2025-08-19 to nightly-2025-08-20
regressed nightly: nightly-2025-08-20
searched commit range: 9eb4a26...05f5a58
regressed commit: 8365fcb

bisected with cargo-bisect-rustc v0.6.8

Host triple: aarch64-apple-darwin
Reproduce with:

cargo bisect-rustc --timeout 120 --start 2025-08-19 --end 2025-08-20 -- doc 

Additional Details

I'd guess this is probably from #144476, CC @notriddle @lolbinarycat @GuillaumeGomez. I see that the regression was spotted by the perf test suite, but probably not to the degree that I'm hitting.

I tried creating a more minimal reproducer for this, but that was kinda hard, since it's a performance thing, so it only really shows up in larger cases :/.

I did try looking at Activity Monitor, it looks like often there's only a single rustdoc process consuming CPU, and only intermittently, so there might be some global locking going on? fseventsd also seemed very active, but that might be normal, idk.