Computers actually got slower to boot after DDR5. It was true that ultrafast boot would boot in something like 5 seconds from post, now auto memory timings are harder to get right and so require more iterations to achieve stability.
Doesn't memory training only happen once when you first boot the machine (or reset bios).
I mean having a lot of ram will take a long time to post, but that's not unique to DDR5. My server is DDR4 with 64 gigs of ram and with it's original CPU it took ages for the post to finish.
Sure, if you're not multitasking and running uninterruptible or PITA-to-get-going-again processes or are just in a good flow where it'll take way more than 20 seconds to reopen all the programs you had running and breaks it
I had to update a Chromebook-like machine that was running Windows not to long ago. It was excruciating. The restart progress bar on one update after reboot took ~30 minutes to reach 3%.
Keep in mind that the computer is unusable during this time, and all it takes is one poweroff to brick the machine. Ask me how I know :) . I had to leave it plugged in overnight to finish.
If this comment is referring to Windows reboots after update, I will call it confidently incorrect.