I'm reading through the #guix manual, and just realized something (I think):
The way Guix sets up `substitutes` means that I could compile everything from source on my powerful desktop* and then set that as a substitute for my laptop—which would then pull down the binary packages from there.
(*NOTE: I don't have a powerful desktop, but I have hopes/plans to get one)
This seems like it's pretty close to the advantages of #gentoo without (as much of) the pain!