Very slow first iPhone and iPad photo sync over usb-a with Catalina

I finally got (most of) my photos loaded... I split them into folders of 500 photos or less and processed them one at a time: I left a small (10 images) folder always on, and I'd add a new 2nd folder and let to process (by leaving most folders off, it processed in "only" 10 minutes). If they loaded okay, I backed up iPod Photo Cache to another folder. If most loaded okay and only a few were black, I moved them to a bad images folder that I don't sync. I then unchecked the new folder (leaving just the sample one) and synced again (this would just take a few seconds to send the delete command to the iPad). If the photos did NOT load okay (lots of black images), I deleted iPod Photo cache and copied the last good backup back into the main images folder. I then rebooted the Mac and iPad (rebooting the MBP is required to get images working for a while again, I don't know about the iPad).

Once I got to my iPhone X photos (.HEIC files instead of .JPG files), I never had any black images. I could have possibly processed them all at once, but I stayed to the previous method (folders of about 500 images each). Once all the photos were processed, I checked sync All folders, which took about 3 tries to get them all copied over.

(All that being said, I haven't dared sync another device with a different screen resolution and start all over again. But also, this time I remembered to back up iPad Photo Cache. (Hint: Time Machine does not back it up.).)