Wow. I went from 372 GB on Sierra HFS+ to 286 GB on High Sierra AFPS. That's a total of 86 GB saved on my 2017 i5 SSD iMac. Now, about 7 GB of that was due to a smaller Applications folder, since I didn't install a bunch of software I didn't need anymore. However, Most of the rest of that was in the various Library folders: /Library - 12.2 GB --> 1.6 GB ~/Library (user 1) - 21.7 GB --> 0.3 GB ~/Library (user 2) - 41.2 GB --> 1.0 GB Total just in the various libraries = 75.1 GB - 2.9 GB = 72.2 GB ---- I hadn't done a clean install in years. It's amazing just how much junk can accumulate in those Library folders, esp. in the user ones which are hidden. It seems the big ones were ~/Library/Application Support and there are folders in there for software I haven't used in 5 years. The biggest single folder was ~/Library/Application Support/Mobile Sync which is the iTunes backup directory. I guess this one will grow over time again, and yes, one would be able to delete those iOS backups via the Manage Storage tab for About This Mac, but at least I've cleaned out the bazillion other unused folders too. Boot time has decreased from ~23 seconds (on a 1 TB PCIe SSD) to 17 seconds. But the best is the improvement in wake-from-sleep. In both Sierra and then High Sierra, wake-from-sleep would take 5-7 seconds each time. It was really irritating, especially considering it would be only one second on my clean installed laptops. After the clean install on this 2017 i5 iMac, wake-from-sleep is now also only 1 second. While it's true deleting iOS device backups would have gotten me most of the space back, I don't think it would have decreased my wake-from-sleep times like this.