Apparently #Gentoo's sys-apps/sandbox (which ensures ebuilds don't make a mess outside of their build "sandbox") had a huge performance regression which caused webkit-gtk build times to go from 9 minutes to 1 hour.
After collecting a ton of data, applying patches, reverting patches, etc, I filed https://bugs.gentoo.org/910273 and it seems we have a fix.
The proposed patch removes the use of the faccessat() function and instead relies on fstatat64().
The system I'm testing on is a 64-core/128-thread beast, and I found that building with -j32 is actually significantly faster than with -j128 (39 minutes vs 1 hour).
So the faccessat() function must be causing some sort of serialization that essentially causes a denial of service with that many jobs?