i suspect it's primarily for systems that lack the performance to compile code itself
Counterargument: compiling the OS by hand bit by bit is in no ways a normal thing to do for a reason, and it's why Windows, Debian and Arch offer precompiled universal binaries. Because the normal thing to do is to offer something that installs and works, not something that's autistically fine tuned to every single machine with it's slightest quirks.
Gentoo may be good when you have a very purpose-specific machine, or a set of those that are all unique, like digital kiosks, so you create a compilation that's meant specifically for those machines with what they're meant to do. If you're building a regular PC or using a regular laptop you're just flexing at that point, or you're using something so ancient that the purpose compiled binaries actually make a difference, by which point the issue is not the distro but the machine, and even a perfect Gentoo install will be shitting itself while you're trying to browse KF, because all modern websites and browsers have a higher resource demand.
The point is: manually compiling OS binaries on a computer you're going to use to browse the web, play games, do taxes and other normal things is incredibly stupid and by that point you're using your PC as a flex of your Linux skills instead of, well, using it as a PC. Debian is enough for most people that want a PC and not a tinker box.
Daily driving Gentoo is for autists who want to constantly be learning about the inner workings of their system and can make use of its extreme customizability.
Yeah AKA people who don't really use their PC for any normal tasks and use them as tinker boxes. I see Gentoo and Arch as tinker distros, too complex and unreliable for any regular use, but great if you want to learn about Linux and how it works, assuming you're doing it on a separate machine where you won't lose anything important when you fuck up the dd command.