I recently set up a Dell workstation with that much memory for a lab at work. The first time I booted it I was afraid that it was dead out of the box. It probably took ~5 minutes to POST and get to the Dell logo. Dells also have this weird thing where they turn on for a couple of seconds after you turn on the power, and it took me half an hour to figure out why it kept shutting down when I tried to boot it.
That's likely the memory training that every modern machine has to do when first seeing new memory (or if the training data is cleared from "CMOS"). Basically they have to discover exactly how tight the timings are for each bank so they can drive the memory efficiently and properly. Timings and latency just didn't used to be so tight compared to the good old days of EDO ram.
Of all my Linux VMs, Alpine Linux boots the fastest--practically instantaneously. Alpine Linux doesn't use systemd.
Parallel init only helps on large, multi-service systems, but those are precisely the ones that (1) rarely reboot and (2) will have relatively long boot sequences, regardless.
i've got servers just out of 5-year warranty with 768GB of RAM factory installed. they tend to take a while to boot but they aren't rebooted often enough to remember how long the POST takes.