I think that if you were to profile the startup of any modern OS these days (with the exception of those that are designed as lightweight OSs), the startup time is not due to starting the OS kernel, but to the identification of system resources and the startup of services.
I occasionally boot an Intel port of Edition 7 UNIX in a VM, and it's noticeable how little is actually running. Just init, cron (which also performs the actions of the sync daemon), your shell (and a ps), lpd (if you're runnning it), and any gettys for any other terminals.
Something like this starts in a flash! I suspect that the system you're describing was similarly lightweight.
Just look at a modern Linux or Windows. There's literally dozens of processes, all of which need to be started, most of which you don't know what they do (especially with obscure, ambiguous naming conventions used by Windows!)
I'm sure that all of these things are needed, but I sometimes wish for the more simple systems of the past.