GrapheneOS said they are working with an unnamed OEM to make devices compatible with GOS. Hopefully they are good, because Pixels may indeed be a dead end.
Because CGNAT means that their router does not have a public IP - just like your router has a single IP that is shared between your devices (using NAT), their ISP shares an IP between multiple customers (also using NAT, it's just called carrier-grade NAT to differentiate who is doing it).
Off the top of my head the compiler is slow because:
With C you can compile every file in parallel, in Rust compilation of a single crate is serial (hence splitting up large projects into many crates is important, but that makes development somewhat more difficult)
LLVM itself is pretty slow
Generic functions are monomorphized (there's a unique machine code version of it for every concrete type combination they are called with) to improve runtime performance, but that gives LLVM a lot more work to do - see point 2
This is a recent example of a problem that required manual intervention or the system would not boot after updates. This happens every now and then on arch, it's why you should check arch news before updating.
Doesn't LTS change like twice a year nowadays?