Your iMac is currently processing the photos in your Photos library, presumably because you’ve just imported/converted an existing Photos library from an earlier version of macOS.
If you suspect this has been the cause of your sluggishness for a couple of days, then it’s most likely you’ve got a very large photo library and that it’s being processed for the first time on your new iMac.
The fact that Photos isn’t running actually fits with your scenario, as launching the Photos app will pause the
Although it takes a long time, once it’s finished your Photos app will be able to perform a lot of advanced functions due to all the metadata etc it’s processed, so I would let it finish what it’s doing.
Should you need to pause the process in order to free up some of your CPU, I would recommend just opening the Photos app for a while and then quitting it when you’re not using the computer. Note, however, that minimising the Photos app will act to restart the
photoanalysisd daemon, so just leave it running in the background while you want to keep the daemon paused.