Originally Posted by
Tartarus You should switch to Ziggo or fiber if applicable. Tele2 and the likes are rubbish.
I assume you're being sarcastic here...

If Sierra NEVER had any issues with the Photos app, the internet connection can't get buggy all of a sudden only because I switched to High Sierra.
(besides, my ISP - xs4all - is named number 1 ISP in our country for years now, probably for a good reason)
• Jokes aside;
I just tested my MacBook... and it has the EXACT same problem as my iMac has.
It just doesn't sync, after 2 hours of 'uploading 48 pics' the progress bar still hasn't moved an inch...
I really think it's a software bug, since it's kinda fishy now that both my macOS systems (iMac/MacBook) have the exact same problem.