Slow iMac after Mojave upgrade

Since I updated my iMac on Mojave just over a month ago, the whole system runs out. It's like working on an old PowerMac from the mid-nineties. You can NOT open any software in less than 3 minutes. However, opening a single file takes a long time or is aroused by the screen saver. It is not necessary to talk about multitasking under these conditions. I have an iMac (retina 5k, 27 inches, late 2014) that I loved earlier. I wonder if there will soon be a solution to this nonsense. It's absolutely not what I've ever experienced with an Apple device. Please make someone … !!!!