I think you are right. I remember hearing it took at least one full minute for the image to set on the negative. Can you imagine holding a smile for a full minute or longer?
Yes, as that photo of first known human image says, it took 10 min to expose. Sure it improved even by the vet’s time, but still long.
I’m not sure I could make myself keep thinking of “Seinfeld’s” “Say Van Delay!” scene long enough to keep smiling for these old sessions!