However, there’s a nasty side effect of turning iCloud off and back on: iCloud Photo Library needs to re-upload all your photos. It does this in order to compare the library’s contents to the synchronization “truth” at iCloud. Fair enough, except that this process can take days, depending on the size of your Photos library and the speed of your Internet connection. Bad Apple! We don’t see that sort of poor performance with Dropbox or Google Drive, and this behavior is both unnecessary and driving people away from iCloud Photo Library.
I’ve had iCloud issues, and, when Apple support suggested I turn off iCloud and turn it on again, I refused. Because the last time I did that, I had to upload some 30 GB of photos, and it took a week. My photo library is now around 45 GB, and I have a 1 Mbps upload.
Adam suggests that not all the data is uploaded, but I watched it cripple my internet access for a week, since I could only allow it to upload overnight.
This doesn’t happen with iTunes Match or iCloud Music Library; they need to fix this.
Source: Bad Apple #1: iCloud Photo Library Re-uploading – TidBITS