In FAI version 5.5.4 the command fai-diskimage now supports raw.xz and raw.zst format and multiple threads when compressing these formats. The command fai-make-nfsroot has a new option -P which also preserves the ssh host keys from inside the nfsroot.
I've also uploaded a new dracut version to the package repository of the FAI project, since FAI uses dracut during the boot process. I will create new FAI ISO images during the next week. -- regards Thomas