The latest OS are slow on my iMac. Opening apps(even settings, in-built apps), open Finder, Preview are slow. Boot consider normal. Im a graphic designer, my majority apps are adobe illustrator & photoshop. The lag are crazy when im performing those heavy apps compared to previous OS High Sierra. Atleast High Sierra are doing fine. Any recommendation for me before i try to do a clean install for it? Spec: iMac 21.5, late 2012. 2.7 GHz Intel Core i5 & 8G rams. (Problem solved) 10 October 2018 Edit: The final solution are make a clean install. problem solved. iMac 2012 now works like a charm.