Any problem in computer science can be solved with another level of indirection. (David Wheeler)
I wanted to learn how to swim, so Google showed me how to turn on the water at the sink and let me splash it around a bit. They then dragged me into a helicopter, flew way out into the ocean and dumped me out. (Tony Lawrence)
iCloud photo sharing is very convenient. Snap a picture on your iPhone and it very quickly is also available on your iPad and on your Mac or Windows computer. For example, this weekend I took 70 or so pictures at a friend's birthday party on Saturday. When I got home, all the photos were waiting on my computer, ready to share with our friends.
However, there's a down side to this, especially when there are large numbers of photos to sync. The syncing is aggressive enough that it may slow down your computer - Apple needs to fix that. There's a possible solution in the links at the end of this article, but that has its own issues. If you find yourself affected by syncing, there is a Pause for One Day button in the iCloud pane of Photos preferences. That can help if you need to reclaim the cpu to get some work done.
I got zinged in another way by this. When we go to our summer weekend getaway, we take my wife's Mac Mini because it is obviously easier to transport than my iMac. We keep a screen, keyboard and mouse there so we only need to take the Mac Mini itself. Of course we each have our own accounts on that machine.
I completely forgot that I had turned on iCloud photo syncing. When I logged into my account there, it seemed slow, but I attributed that to the slow wifi provided there. I switched to the Verizon Jetpack that I have for when I need to do customer related work. I proceeded to do the work I needed to do but was shocked to see that I had consumed neatly 6 GB of expensive JetPack data when I finished. That seemed odd to me, but I shrugged and simply bumped it up at Verizon.
A few days later I needed to do more work, so logged in again. This time the toll was another 2 GB, which made absolutely no sense as I was only using an ssh connection to edit some program files. How could I possibly be using up all this data?
Of course iCloud was the reason. While I was typing a few hundred characters, it was busy syncing thousands of photos to the Mac Mini. It doesn't HAVE to do that, but the default setting is Download Originals to This Mac. You can change that to Optimize Mac Storage, but even that can still suck up your bandwidth.
I know better now: before we run off, I log in on that machine and make sure it has finished syncing before I pack it up. It is a good idea to have local copies on one or more machines; otherwise you are depending entirely on Apple to keep your photos safe.