Adam C. Engst, TidBits:
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.
It turns out that there are quite a few actions that can cause your entire Photos library to be re-uploaded to iCloud […]
I keep over 350 GB of photos stored in iCloud, which would take me nearly a full week to re-upload — assuming I did not want to do anything else on my home internet connection at the time.
This is an especially bad behaviour because toggling iCloud is one of the first troubleshooting steps you’ll encounter when you try to fix anything related to iCloud or syncing. Oddly enough, both behaviours should be fixed: iCloud Photo Library should be able to more efficiently compare local and remote libraries, and toggling iCloud should not be seen as a troubleshooting step due to how interruptive it is.
Update: Whether your iCloud Photo Library is entirely re-uploaded is unclear at the moment. Engst says that his re-sync was faster than the initial iCloud Photo Library upload, so he’s not sure that it’s re-uploading everything. But Apple’s documentation indicates that it is a “re-upload”, and Kirk McElhearn (via Michael Tsai) also says that his library was re-uploaded as well.