I found your answer helpfull and was able to further search the asus-forum with your help..
I found https://rog.asus.com/forum/showthread.php?96482-Cold-Boot-Issues
"My understanding of MRC Fastboot is that you are skipping memory training if you enable it. You only want to do so once you are sure your memory settings and sticks themselves are stable so try disable first. MCH Fullcheck will cause the system to reboot, retraining the memory until it is stable. Disabled will likely result in a failure to post if something is causing instability."
by @redone13 post #6
Will definitely double check that today evening.
Ryzen/Threadripper is slow to POST. Regardless of Fastboot setting used in UEFI and or Window Fast Startup.
Even with 1x GPU and SATA drive I can see ~50sec POST time from pressing power and getting to desktop.
If AC is removed from motherboard prior to POST then it will do a POST to set onboard ECs then turn off and do another POST before it get to OS. So even slower.