SD cards can’t be slow in this setup. You are far better to just import from the SD card, then extract the card on finish.
Clean it up in camera, usually the fastest method, or plug it into your computer and delete the images with Finder.
Import the photos to a folder on your Mac. Then you can use smart folders and spotlight and fast file operations to delete / move / Pre-process the files.
Photos is absolutely not a folder oriented Storage system. Check out Photo Mechanic by camera bits for a folder oriented workflow.
We use that and high speed SD card readers and can ingest about 100,...
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.
Is there a reason you want/need to do this via the Finder, exploring the Photos Library.photoslibrary package database?
If you are going to do that, why not just "import" the files from your SD Card using Finder? Skip importing via Photos entirely.
If you are importing into Photos, I'd suggest you also export via Photos.app rather than exploring its folder ...
It seems that my iPhone were full in some way: after removing gobs of photos and videos it is working to sync. I did not see/notice any indicator of the memory being full and the Settings|General|Storage had said 8.7GB out of 16GB before deleting anything (now says 6.6 out of 16GB). That is strange behavior.
To answer my own question:
I felt brave and fired up a 3rd party tool that could access the iPhone filesystem. In the directory Media I renamed the directories PhotoData to PhotoData_old, DCIM to DCIM_old and Photos to Photos_old.
Then I restarted the iPhone. After reboot completed I checked the Media directory again. PhotoData, DCIM and Photos directory ...