I couldn't find any mention of this consistent behavior, let alone the fix for it, so I'm posting both.
You can see the folders and files within the Mac's "Photos Library.photoslibrary" package, in your Pictures folder, by right-clicking and selecting "Show Package Contents". This has been true for years, so when you want to get an old folder of Masters or Originals from an older version of Photos, perhaps on another device, the simplest thing has been to grab the whole folder, for the date you want.
Now, I don't know how long this has been going on, but the current version of Photos won't import this folder properly. If you select such an old Photos sub-folder when you import, a folder which has full-resolution originals, it will consistently give you new low-resolution images in your new Photos library. And it takes extra-long to do this unhelpful import, probably because it's generating those low-rez images.
I have a suspicion that this behavior is related to the backwards-incompatible iCloud rollout, since that's the only situation where I'd imagine a low-rez image generation would be useful. But Photos will import badly from these old folders even when iCloud is turned off.
It should just find the .JPGs and import them. But it's confused by other files and folders in the folder.
The solution is simple: do a search for the .JPG files, select them all, and copy them into a flat folder.
Photos will import all the full-resolution photos from that structure, with no problem.
Although only indirectly related, you may notice this behavior when your iCloud is having trouble. That little cloud with an exclamation point (not the user's fault ... Apple's fault) in the lower right hand corner of the low-rez photo, which, when you click on it, tells you that "An error occurred while loading a higher quality version of this image.” If you see a low-rez image, it could be caused by all those problems that people mention online ... but it could also be the problem I mentioned here: you imported from an old file structure that Photos, for no good reason, cannot handle properly.
