Huh? I can do a full kernel rebuild in ~40s on a 3900X.

Maybe because Arch has more modules enabled than the default config?

without running clean?

cause that's not even in the right ballpark for a stripped kernel config

Yes, in a clean tree that just got cloned, and the result does boot.

20 years ago, it took me only around 10 minutes on a single-core 32-bit CPU, with 1/1024th the RAM, and spinning rust for storage. How is it not even twice as fast today?

Maybe better and more time consuming optimizations? 2 mio LOC vs. 26 mio LOC probably doesn't help either. Maybe there is some bottleneck somehwere in the software or hardware?

I have no sense of how long that is. What would it take on a 4 thread Macbook Pro?

I have not tried installing Linux on a more recent MacBook Pro (USB-C generation) so I couldn't tell, but I remembered it took around an hour with Arch Linux's default config (e.g. "let's compile this and go have a breakfast & make a coffee and hope it's done")

Went to lunch and timed a compile of the linux kernel (v3.19) (default options) at 14 minutes on my 13" 2015 macbook pro. (3.1 GHz Intel Core i7, 16GB RAM).

depends which four thread MacBook pro...

as a rough reference, it took about 35 minutes to build the linux kernel on my xps 13 a few years ago. that computer has a 2C/4T kaby lake processor. your macbook pro might be a little faster if it doesn't have one of the ultra low power CPUs.