iCloud & iCloud Photos · iCloud Photos and the Photos library
How do I pause iCloud Photos syncing on my Mac?
Short answer: At the bottom of Photos' Library view, click Pause to hold syncing (usually for a day) and Resume to continue. Pausing removes nothing; to stop for good, turn off iCloud Photos in Photos → Settings → iCloud.
In Photos, scroll to the bottom of the Library view, where the sync status appears, and click Pause (usually for one day). Click Resume to carry on sooner. Pausing doesn't turn off iCloud Photos or remove anything; it only holds uploads and downloads for a while.
Pausing is useful on a slow or metered connection. To stop syncing entirely, turn off iCloud Photos in Photos → Settings → iCloud — see what happens if you turn it off.