wdsutil /set-server /DefaultX86X64ImageType:x64

https://bjtechnews.org/2014/11/29/speed-up-pxe-boot-in-wds-and-sccm/

Bcdedit -store c:\default.bcd -set {68d9e51c-a129-4ee1-9725-2ab00a957daf} ramdisktftpblocksize 16384