I have used Windows 98, and observed that its shutdown is very fast. How is that?
Shutting down a computer involves two major tasks: stopping all running processes, and ensuring all pending writes are flushed to disk.
Once any running applications have stopped, there isn’t much in the way of system services to stop before the system can be shut down, so Windows 98 does shut down quite quickly compared to current systems.
The same is true of other non-server operating systems of the period; a default installation of OS/2 Warp in particular boots and shuts down very quickly.
PCs of the time had much less resources than now (a typical hard drive of the period would fit several times over in current systems’ RAM, and typical Windows 98 computers had two orders of magnitude less memory than many current systems), so there was far less data to potentially write out before shutting down too. Windows 98 is designed with such constraints in mind, so even on a modern system (well, one where Windows 98 will run) it won’t keep much in its disk caches; it never did keep many pending writes anyway.