Nixos will use/download cached binaries that are available in its repo. It has one of the biggest repositories of any Linux distro. It’s on par with Arch with around 90 thousand packages.
Unless you are doing something custom or niche, your nixos won’t have to compile anything.
Yes, all packages in nixos are available as binaries to download.
The comparison with Arch was just in terms of number of packages. Not the binary availability.
At the bottom of this page, they say that binary cache is currently at 120TB. https://nixos.org/community/index.html
If packages being available as binaries is the main criteria, nix has you covered there.
The biggest issue for most people with Nixos is the learning curve just because it’s so different.