I am experiencing this when "batching" a colour coding.
I use two cameras and like to tag one's images with blue and the other green. I ingest all the images, use control+f to identify images from one camera, then control 3 or 4 as appropriate to colour tag images, and it can take ten or fifteen minutes to colour tag 600 images.
Im on Tahoe, I know I should have waited, but I didn't....
If it continues Ill contact support.
UPDATE: The comments above refer to slow colour tagging in Nikon jpg files. Strangely colour tagging the same number of corresponding Nikon raw files is almost instantaneous. Curious.