I do not know about the other issues, but I imagine most pictures taken nowadays, compared to the early 2000s, are in a much higher resolution than what is possible to process in a very fast way (without HW acceleration). The only way is to watch pictures being added to the disk and process them ASAP, which is a bit more involved than many users like (and adds hidden costs to the simple "copy" operation).
Aren't thumbnails already embedded in the picture metadata usually? I don't know if they actually need to process every single picture to generate it, so the size should not be that bit of a factor. I'm not sure how nautilus deals with pictures though
There may be done image formats with thumbnails in metadata, but in general, no. You have to generate them. Which makes sense too since you'll need multiple sizes of them, depending on your use case.
