If you’ve come here looking for a solution to your Mac locking up everytime you enable iCloud Photo Library, you’ve come to the right place.
You’re probably enabling it for the first time and your system keeps freezing for some unknown reason, either not being able to wake from sleep or going completely unresponsive.
The same thing happened to us. After pounding our heads against the wall for a good week, we noticed the library had all manner of weird files in there like AVI and PSD. Photos did not like this one bit as it to tried to convert them during the upload process.
You’ll need to manually identify and remove these files individually in order for the upload process to succeed. If you’ve removed them all and it still freezes, you probably still have files in there Photos doesn’t like.
Once they’ve all been removed, Photos should sync and upload the remaining files.