slow 2019 imac 21.5 inch retina 4k

When still under warranty when it had to be repaired, I got it back and before using had to leave town for several months and warranty expired. When got back was running very slow and continues extremely long start up and program loading, have 16 GB ram. When checking activity monitor cpu shows red but no programs show red. I've put up with it for a couple years, have older iMac and it continues to function well. Any ideas about what to check / look for / do?