I am trying to open a 160GB Aperture Library with the Photos.app The library can be opend with aperture w/o problems. As soon as I start opening it with Photos, Gatekeeper starts checking the files. This seems to take very long. I did let it running for a couple of hours and quit everything.
I tried to disable Gatekeeper with
sudo spctl --master-disable
But that didn't change a thing. It still opens the dialog and start checking the files, with no progress: