Fast forward 11 months to Dec 2024. I'm having the exact same problem. Found this thread via Googling
mac photos libary "downloading originals" slow.
This is after several weeks of battling this thing. I've already seen the same info in the 50 posts on this thread in different forms, here and there on the web, but this thread does a great job of pinning all the potential workarounds in one place.
The advice here to "Export originals" is what has finally worked for me:
The magic words in blue. OMG what a relief.
It didn't even take that long! Apple Photos might suck at some things, but it does not suck at exporting photos. I was able to queue up 12 parallel exports (1000 to 50000 photos each) within 20 minutes.
The results are still landing on my external SSD but everything looks good.
Important part is that the export process seems to have been enough to nudge the iCloud sync into finally completing!
Here are the steps I took:
- New MacBook Air account "Fotos", with Administrator privs
- Sign into the new "Fotos" Mac account.
- Sign that "Fotos" Mac account into my SAME old iCloud account.
- Start Photos.app, let a couple download. Then quit Photos.app.
- Plug in the shiny new external SSD (this cost more$ than I was originally willing to spend, now seems worth it)
- Move / Copy the newly created (small) Photoslibrary in ~fotos/Pictures to the external SSD
- Doubleclick that moved / copied PhotosLibrary to open Photos.app
- Settings -> "Use as System Library"
- When Prompted “are you sure, any photos not synced”… click Yes sure
- Settings -> "enable iCloud" and check "Download Originals to my Mac"
- Wait ~2h.
- Photos App should show "# of items" increasing quite quickly... these are initially thumbnails and database entries
- "Photos Library.photoslibrary" should grow in file size
- Select All + "File -> Play Slideshow"... that didn't work for me. Maybe too many photos.
- Wait 3+ days for “Downloading originals” to stop... it's still going... "Downloading 22 originals". For >12 hours.
- Tired of this. Started the “Photos Export Experiment”. Success. Results screenshotted above.
Progress graph of step 12 maybe TMI. The flat blue part in the upper right was what I was getting tired of watching not grow.
Anyways, thank you folks on this thread who posted all the helpful hints!