lassulus: 
```
nixpkgs.overlays = [

    # change the compressor for initrd
    # an overlay is used intentionaly since setting the compressor
    # in belows makeInitrd calls, doesn't work reliably
    # (some initrd was still built with wrong compression)
    (curr: prev: {
      makeInitrd = args: prev.makeInitrd (args // {
        compressor = "${pkgs.pigz}/bin/pigz -9";
      });
    })
```