I am trying to create a shared album in iCloud, containing about 750 photos and 50 short videos.
Using Photos both on iOS 9 and El Capitan, the process of adding the photos to the album is extremely slow.
On iOS, when I try to share the photos I am shown a modal "preparing" dialogue that doesn't seem to progress.
On my Mac (which does not have "Optimize Storage" enabled) the sharing progress bar fills slowly, then stays mostly filled for a very very long time, while heavily using the CPU.
Given the amount of network traffic that is caused by the request, I assume that Photos is trying to download/re-upload each image.
Given that the photos are already stored on iCloud, I would expect to be able to share the photos instantly, as I can do with Google Photos.
Is this the expected behaviour or am I suffering from an abnormal issue?
EDIT: added photo of iOS dialog
Adding one photo to a new shared album works ok, but as I add more photos in one go the time taken scales up quickly.