I created a DIY Fusion Drive by adding a ssd to my imac and now find it is slowing down dramatically . . . sometimes to a crawl with a catch-up 2 minutes later. Anyone have any ideas. It has been working beautifully for about 2 years.
Check to see what file system you have and if it is APFS my advice is to restore your system back to Sierra from, hopefully, a back you made before updating. Then I would stick with Sierra until Apple have fixed the problem caused by High Sierra to some Fusion drives.
You could have a look at this and see if it's an option for you: -