On crates.io's codebase at 1fc6bfa on macOS Catalina 10.15.1, 3.1 GHz Dual-Core Intel Core i7, 16 GB RAM:
Full build
$ cargo clean && time cargo +nightly-2019-12-18 build
real 10m1.042s
user 18m47.605s
sys 1m50.826s
$ cargo clean && time cargo +nightly-2019-12-17 build
real 8m33.007s
user 17m14.295s
sys 1m38.205s
Soooo parallelism made it slower?
Full release build
$ cargo clean && time cargo +nightly-2019-12-18 build --release
real 18m58.546s
user 44m1.749s
sys 1m48.858s
$ cargo clean && time cargo +nightly-2019-12-17 build --release
real 16m54.551s
user 43m2.278s
sys 1m38.579s
Also slower for me 
Full check
$ cargo clean && time cargo +nightly-2019-12-18 check
real 6m35.014s
user 12m18.504s
sys 1m22.026s
$ cargo clean && time cargo +nightly-2019-12-17 check
real 5m16.316s
user 11m27.044s
sys 1m17.450s
I'm happy to run anything else that would be useful, or provide any other info, let me know what!