As far as I know they do a few things (but it is hard to find a comprehensive list), including build packages for newer microarchitectures such as the aforementioned x86-64-v3. The default on x86-64 Linux is still to build programs that work on the original AMD Athlon 64 from the early 2000s. That really doesn't make sense any more, and v3 is a good default that still covers the last several years of CPUs.

There are many interesting added instructions and for some programs it can make a large difference, but that will vary wildly from program to program. Phoronix has also done some benchmarks of Arch vs Cachy, and since Phoronix Test Suit mostly uses it's own binaries, what that shows is the difference that the kernel, glibc and system tuning alone makes. And those results do look promising.

I don’t want to spill some memes worth Arch elitism here, but I just doubt Arch derivatives crowd knows what x86-64-v3 thing is. Truth be told, I barely understand that myself.

I think you just did show a lot of elitism and arrogance there. I expect software developers working on any distro to know about this, but not necessarily the users of said distros. (For me, knowing about low level optimisation is part of my dayjob.)

Also, for Cachy in particular they do seem to have some decent developers. One of their devs is the guy who maintains the legacy nvidia drivers on AUR, which involves a fair bit of kernel programming to adapt to changes in new kernel releases (nvidia themselves no longer do so after the first year of drivers becoming legacy).