iCloud & iCloud Photos · iCloud Photos and the Photos library
Why is iCloud Photos taking so long to upload my library?
Short answer: The first upload sends every original in full, in the background, and slows or pauses when you're busy, on battery or on a weak connection — big libraries can take days. Keep the Mac plugged in and awake with Photos open; progress shows at the bottom of the library.
The first upload of a big library can take days: every photo and video is uploaded in full, and Photos does it in the background, slowing down or pausing when you're busy, on battery or on a limited connection. Videos and large libraries take the longest.
To help it along, keep the Mac awake and plugged in on a good connection with Photos open, and don't pause it. Progress shows at the bottom of the library view. Your photos are safe on the Mac while it works — but if they exist only there, keep a backup until the upload finishes.