Whenever I export raw files to jpegs, if Add to This Catalog is selected the import/add operation is very slow, 1 to 3 seconds per image. Images start to get added during the export operation but this can go on for several minutes (think 10 to 20 minutes) after the export is finished. If I uncheck that box and synchronize the folder after the export operation is finished it will import hundreds of images in seconds.
Not stacking, just exporting to same folder. Sometimes with modified file name, sometimes just with the .jpg extension.
LRC 10.1.1, macOS 11.1 Big Sur, clean install this week. But the same thing was happening with High Sierra and LR 9. All images stored on local drives, nothing in the cloud. Happens with TB3 SSD (WD Black, Samsung X5) or internal fusion drive.
There are lots of threads in the forum over the past couple of weeks of problems with iMac, particularly with older units from 2014 and 2015, with LrC 10.1 and 10.1.1. Try and revert to 10.0 and see if that helps until there is a fix from Adobe. They are aware of the problems. See the link below for more info.