I did some testing with TinyCore Linux 10 (kernel 4.19.10) on the same hardware: it boots in about 28" (compared to PK ~60"), and its GUI Shutdown correctly powers off the computer ...
Strangely, I tried pressing Ctrl-Alt-Del -> Shutdown during running the Porteus Kiosk Wizard (having booted the Porteus-Kiosk-4.8.0-x86_64.iso from a USB stick) and it powered off the system correctly, just as TinyCore did! It also correctly detects the press of the Power button and brings up PK's [Shutdown/Sleep/Reboot/Cancel] window.
The Linux boot warning/error message I noticed (both in PK and TC and both when booting from USB and when booting from ssd) is:
Thank you very much in advance for your help,
Code: Select all
[ 0.004397] do_IRQ: 1.55 No irq handler for vector