AFPS

In High Sierra, the Mac follows iOS, tvOS and watchOS devices in transitioning to the Apple File System (APFS). According to Apple, its new file system is designed for flash storage, to be responsive, and to be secure. The second point is evident when performing actions like duplicating a bunch of files, or finding the size of folders. Tasks that may have taken some time to complete on an older version of macOS now often happen almost instantly.

That said, you should approach any major change to a file system with caution. At the very least, ensure that you make a fully bootable clone of your Mac (using the likes of SuperDuper! or Carbon Copy Cloner) before upgrading, just in case things go wrong. But you were planning on doing that anyway, right? Find out more about AFPS on p54.