I have a late '15 27" iMac. I spec'd it from the factory with 16GB RAM (4x 4GB sticks). I've now upgraded to 32GB using Crucial 4x8GB sticks. Now it's crawling, mainly in Adobe apps. Nothing on the machine has changed bar the RAM. All four slots are populated with identical 1867MHz DDR3 sticks and the system reports show all are operating OK. I expected far less in terms of system overheads to run multiple apps (the 16GB setup wansn't slow by any means) but it beachballs like hell now!!!