APFS

High Sierra follows iOS, tvOS, and watchOS in moving to Apple File System (APFS). Apple says this new file system is optimized for flash storage, with responsiveness and security in mind. Responsiveness benefits are clear when doing things like duplicating a bunch of files, or finding the size of folders. Tasks that may have taken a while to complete on older versions of macOS now often happen almost instantly.

That said, approach this change with caution to protect your data. Ensure you have a complete backup of your Mac – ideally one that can be used as a startup disk, made using the likes of SuperDuper – before upgrading, in case things go wrong.