This depends heavily on your security model... Are you trying to use just a literal 'usb key' as a fob for booting the computer? Decrypting the LUKS key will t

This depends heavily on your security model... Are you trying to use just a literal 'usb key' as a fob for booting the computer? Decrypting the LUKS key will take some time, but you can ship the key for decrypting that in the initramfs that you'd store with the kernel on the USB drive.