Mine only took about 10 minutes. This made me curious, so I read through the install log /var/log/install.log - to see yours just open Terminal set it to full screen and hit cmd + a few times until the font is large enough for you to read and then type in a command like: tail -n 2000 /var/log/system.log | more
Reading carefully through that I learn things that will make me do things differently next time. For example, I don't usually use my normal userid to do upgrades as I have a separate admin account for that kind of thing. Well today I found out that it hadn't upgraded my photos or mail libraries because it thought they hadn't been used for a long time, oops.
Anyway, this upgrade updates many app sandboxing configurations, the EFI stuff, the boot and recovery partitions, base system caches and registers some hidden opendirectory nodes, amongst other things. Have a look at the install.log if you're interested.
The upgrading of photos and mail libraries could explain why it's taking so long for some people.
Thanks for the information; so this update is definitely modifying things well outside of Safari.
It's alarming that Apple seem to not be interested in patching El Cap and Sierra for Meltdown, and what I assume in the changes you noted are other mitigations against Spectre at the system level.
Neither of my main machines are on High Sierra due to the the issues I've read about, but now it is feeling like a forced upgrade. I can at least do that on my MBP, but my old El Cap Mac Pro I still use for audio work is now in a state of questionable security without even the Meltdown fix.