Maybe it's the single slowest individual item, but it's very far from being a significant fraction of boot time. And the capacity really hasn't kept up the way speed has. My desktop has 24GB of DDR3 1600 and manages to post in under 2 seconds. And that's pretty old by today's standards. Mid level modern hardware runs at least a circle or two around this system in terms of speed, but in terms of capacity it's still right in line with a higher end system today. Maybe I'm atypical but my boot time is dominated by my OS spinning itself up, by a long shot.
I suppose it depends. My AMD DDR5 machine spends most of its boot time on memory training. Once that's done it's only a few seconds into the OS. (I know I could enable fast boot to skip that most of the time, but I rarely reboot and would rather have the guaranteed stability.)
