Strangely, after having this problem and turning off iCloud sharing on my Mac for several days, the problem has gine away. I have added several thousand photos to my Photo library since then, waiting until I have imported them all to my local library including adding date, location, description, faces and other such information, including creating albums, before finally turning on the iCloud sharing feature and in less than an hour the iCloud library was updated and showed the same number of photos as my local library with no apparent internet connection issues. I have had albums disappear in the local library and then reappear, and other strange issues but the "I'm breaking your internet connection" problem has not reappeared so is inconsistent.
However, once the iCloud library was updated, my iPhone drained its battery very quickly downloading the updates (even with the optimize storage feature enabled) so I recommned keeping Photo syncing off on mobile devices if you make large library updates, turning it on only when your device is charging.
And BTW, I love that Apple Mail treats updates from its own support communities as junk mail. 🙂