Startup times for desktop operating systems seem to have trended up, then down, between the '80s and today; with the worst performance being in the late '90s to 2000 or so when rebooting on any of the major systems could be a several-minutes affair. Today, typical boot times for Mac, Windows, or GNU/Linux systems can be in a handful of seconds if no boot-time repairs (that's "fsck" to us Unix nerds) are required.

I know that a few years back, there was a big effort in the Linux space to improve startup times, in particular by switching from serial startup routines (with only one subsystem starting at once) to parallel ones where multiple independent subsystems could be starting at the same time. I expect the same was true on the other major systems as well.