High Sierra uses a new bespoke FileSystem APFS (God save us from Apple's own filesystems again).
It seems to be very much SSD orientated and many older machines or non SSD machines just seem to Grind unbearably.
That, and the Secure Token FileVault issues are the main reason why I am avoiding High Sierra like the plague until all the kinks are well and truly worked out.
If you are committed to using High Sierra I'm afraid you may just have to take the pain or look at your hardware.
I'm stuck using a Mac Mini that's a little slow on Sierra, I had High Sierra on it for 2 days and downgraded it again (over 50% of those 2 days was spend doing the upgrade and just trying to log in and do day to day tasks) thankfully a rebuild back to Sierra only took an hour.
