I'm comparing it to my M2 laptop, but in practice the 9955HX is substantially faster than even the M4 Pro I have in my Mac Mini, about 30%~ or so in wall clock time for Rust compilation.

Yep, Pro only has 12 cores, and a third of those are efficiency cores. Even the Max loses some of its performance to efficiency cores. This is why I was so upset to see Intel replace a bunch of performance cores with efficiency cores. (Remember how Intel used to offer enthusiast chips with up to 18 full fucking cores? Now they think 8 full cores + 16 small useless cores is the answer? I am appalled. Even aside from HEDT they used to offer up to 10 full cores.) More, and more performant, hardware threads is almost always the path to faster Rust compilation. Lose a few of those to efficiency cores and even Apple can fall behind.