Actually, -A makes it faster and more precise about what you want to install. The Nixpkgs repository is defined as large associative array containing packages. When "-A nixpkgs.ripgrep" is specified, nix-env looks up the associative array using the key "ripgrep". That in theory should take constant time regardless of the size of Nixpkgs. When -A isn't specified, nix-env searches the entire associative array for a package whose "name" attribute looks similar to "ripgrep." That takes more time as the size of Nixpkgs grows. It's also less precise because the "name" attribute for packages aren't guaranteed to be unique, unlike a key for an associative array. It doesn't do an exact match either, because the "name" attribute also includes version numbers by convention.

For uninstalling, nix-env doesn't have the -A flag, because it has to operate on the set of installed packages, not Nixpkgs.