Actually I thought of this already. On several of my cards that work very slowly in the SD slot, I tried adjusting their position in the slot slightly, pushing in, out, up, down slightly, thinking it might be a bad contact. Made no difference. All work perfectly in three different card readers without hesitation. Also does not explain why the card is recognized, Finder sees all the files, but takes forever to read a file. 100 photos take half an hour to copy.
Cards are sometimes a problem. I have had two full size SD cards fail, and about five Micro-SD cards fail. This is in about 50TB of recordings. All are the "extreme" or "best" SD cards from brand names like Sony, Samsung and Sandisk (and bought from reputable merchants like B&H in order to avoid fakes).
Yes, those free Sandisk microSD adapters that come with the microSD cards are garbage.