Slow Wheel of Death

For about six months, my iMac (21.5 inch 2017) has been sluggish. The wheel of death appears almost all the time, whatever I do and whether or not I have even one app open. It even has trouble opening apps in Safari where typing lags behind on the screen all the time. I have done all updates and have taken the desktop to Apple store locally twice to troubleshoot. They reset my Mac at the store which took about two hours and then I restored it manually about two months ago, not through my external drive out of concern there might be a corrupt file somewhere. I have tried Clean my Mac app recently and that made no difference. There is no malware on the Mac and I run Malwarebytes often. I have super fast internet connection and it's hardwired. I'm afraid this is a lemon and not sure what to do next. Really has shaken may confidence in Apple.