What does photolibraryd do whenever I log in to my Mac? Whenever I log into my MacBook Pro the CPU cranks up to 100%+ for a few seconds, my board gets hot, and the fans start going.  I open up Activity Monitor to find that the culprit is 'photolibraryd'.  What's going on?  What's it doing that uses so much CPU?
It also matches photos that are on your local library and in iCloud. So for instance, if you have a sync problem, you can toggle iCloud Photo Library in Photos app → Preferences → iCloud and this will cause a complete re-sync of the local and the iCloud photos.

When this happens (at least on my 51K photo library), it takes 24 hours or so of photolibraryd running to match photos and sync the changes.