New Photos App Sync Hangs after mass import

Hi,

I recently updated my MacOSX to 10.10.3 and installed the Photos App. After few minutes exploring the app and configuring the sync with iCloud, i started importing a mass volume of photos of my external HD. They are all imported successfully (6337 photos of a old library).

But, i notice that syncing is not working. Past two days my Macbook was on and nothing changes.

Any ideas?