Note: This was written for macOS Yosemite and older Macs. If you're visiting this in the future, it may not help as much for current issues you're having with iCloud Photo Library syncing.

If you've started using the new Photos app for Yosemite and you have a slighter older Mac, you may notice your fans spinning up and your computer getting hot even though it seems like you're not actually doing anything.

As best as I can figure out, the culprit seems to be the syncing service running in the background to keep your Photos.app in sync with what you have in iCloud Photo Library.

I believe "cloudd" is responsible for all iCloud syncing, including the possible thousands of photos being flung from Mac to iPhone to iCloud. "com.apple.photos.VideoConversionService" is likely doing exactly what it describes - any videos that aren't in a compatible format for iCloud are being converted in the background while you're trying to get other work done.

How to Use Your iPhone as a Webcam

https://www.youtube.com/watch?v=JYLFDJCp3MA

The blessing with Apple's iCloud syncing system is that it's all behind the scenes and there's nothing to configure beyond checking a box and signing in with your iCloud account. The curse is that Apple's iCloud syncing system is all behind the scenes and there's no way to know if it's going to be done soon or way to pause it and restart later like you can with a service like Dropbox.

I'm sure syncing is especially rough right now as millions of people are all throwing their photo libraries at iCloud at the same time. The scale of it is impressive, scary and interesting.

With iCloud Photo Library there's one less thing to worry about backing up if you switch to a new Mac - now you can just sign in and your photos are all there ready to keep using. As long as Apple keeps building data farms to keep up.