imac – Mojave problems …!

Since I upgraded my iMac to Mojave, a little over a month ago, the whole system ran out. It's like I'm working on an old PowerMac from the mid-nineties. You can not open ANY software in less than 3 minutes, even open a single file takes a long time to do it or wake up from the screen saver, it is not necessary to talk about performing multiple tasks in these conditions. I have an iMac (Retina 5k, 27 inches, late 2014) that I used to love. Wondering if there is a solution for this non-sense coming soon. It's not quite what I've experienced with any of the Apple devices that I own. Please someone do something … !!!!