Exact same problem as all others with iMac & Fusion Drive. (Late 2017, 27" 5K Retina, 24GB Ram upgrade). Its running slower than the 12 yr old macbook pro it replaced....was brilliant on previous OS which is the really annoying thing.
NMcA
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
From
Scotland