I have Photos creating a shared album. It seems to upload five or six photos, wait ten to fifteen minutes, then upload some more.
Judging by web search results, this is new behavior (or limited to my situation). Many hits of people wanting to know how to slow it down; none (in the first page of results) complaining that it is slow.
Has Apple recently done something to limit the Photos bandwidth?
A similar question seemed to have an answer. The difference is that they were sharing photos already uploaded, while I was sharing new ones.
But if Apple is merely resizing them locally, and then uploading, the speed is still unexplained. GraphicConverter resizes an image in one or two seconds, not ten/twenty/thirty. And an upload of a 2048-pixel wide photo should be fast as well. (After all, images taken by an iPhone appear on other devices as fast as I can open the device.)