Why is delete items after import so slow? I am importing thousands of photos from SD cards into albums on my Mac. Import is not super-quick but OK, but when I select "delete items after import" then it seems to delete about one image per second. Further, it claims import is complete but blocks more updates until complete, which gives the impression of hanging.

Is this a known issue and is there any explanation or workaround?
Use image capture to get the photos to your Mac. Then import from a fast storage location. The delay is cleanup data to the SD cards. 

Get all the data off fast so you can let photos import rapidly and decouple the delete of the card till later.