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Up until macOS 10.12 Sierra iCloud Photo Library poorly managed available resources while synchronizing its database. More specifically, the Photos app would consume all available bandwidth, rendering your internet connection unusable. The result is a terrible synchronization performance.
Apple has considerably improved its iCloud Photo Library since it became available as part of OS X 10.10 Yosemite and iOS 8. As a former Aperture user, I did have some concerns that I managed to work through, and I found workarounds for issues like sharing libraries with family members. Nevertheless, iCloud Photo Library has a few remaining issues that won’t impact you on a daily basis, but when they do, they are a major pain in the butt.
Restoring an iCloud Photo Library from Backup
One of those issues is related to restoring an iCloud Photo Library from Backup.
Last weekend, I re-installed OS X 10.11 El Capitan on my iMac, hoping it would fix some of the bugs I had identified. Due to its size, I store my iCloud Photo Library on a LaCie 5big Thunderbolt 2 Raid. So re-installing OS X on my internal hard drive had no effect on my iCloud Photo Library. However, when I tried to open the library after the re-install was complete, Photos claimed that it had to delete incomplete items from my library and re-download them. The result was that Photos had to re-synchronize my complete library. That leads to a second and even more severe problem.
Synchronization Performance is Horrible
Synchronizing an iCloud Photo Library consumes all available bandwidth, making every other application that depends on your internet connection virtually unusable. My iCloud Photo Library is about 300GB large and synchronizing it takes a couple of days. I have a Comcast Business Class 150/20mbit internet connection, and Photos just kills it. Not even web browsing works properly while Photos is synchronizing. So I ended up pausing and resuming the synchronization process a lot. The problem is that not even at night we can afford to let Photos suck the life out of our internet connection because we use it to stream Netflix and other content via Apple TV.